% --------------------------------------------------------------------------
% the MODIAGRAM package
%
%   easy creation of molecular orbital diagrams
%
% --------------------------------------------------------------------------
% Clemens Niederberger
% Web:    https://www.bitbucket.org/cgnieder/modiagram
% E-Mail: contact@mychemistry.eu
% --------------------------------------------------------------------------
% If you have any ideas, questions, suggestions or bugs to report, please
% feel free to contact me.
% --------------------------------------------------------------------------
% Copyright 2011--2020 Clemens Niederberger
%
% This work may be distributed and/or modified under the
% conditions of the LaTeX Project Public License, either version 1.3c
% of this license or (at your option) any later version.
% The latest version of this license is in
%   http://www.latex-project.org/lppl.txt
% and version 1.3c or later is part of all distributions of LaTeX
% version 2008/05/04 or later.
%
% This work has the LPPL maintenance status `maintained'.
%
% The Current Maintainer of this work is Clemens Niederberger.
%
% This work consists of the files modiagram.sty, modiagram_en.tex,
% README and the derived file modiagram_en.pdf.
% --------------------------------------------------------------------------
\RequirePackage {expl3,xparse,l3keys2e,tikz,chemgreek}
\usetikzlibrary{calc,arrows}
\ProvidesExplPackage
  {modiagram}
  {2020/11/08}
  {0.3a}
  {easy creation of molecular orbital diagrams}

% --------------------------------------------------------------------------- %
% error messages
\msg_new:nnn {modiagram} {missing-atom}
  {
    You~ need~ to~ specify~ the~ left~ and~ right~ atom~ first~ before~
    you~ can~ use~ \token_to_str:N \molecule \c_space_tl \msg_line_context: .
  }

\msg_new:nnn {modiagram} {missing-AO}
  {
    You~ need~ to~ specify~ both~ #2~ AOs~ first~ before~ you~ can~ use~
    #1~ \msg_line_context: .
  }

\msg_new:nnn {modiagram} {electron-placement}
  {
    The~ value~ for~ the~ electron~ placement~ must~ be~ between~ 0~
    and~ 1~ \msg_line_context: . 
  }

\msg_new:nnn {modiagram} {deprecated}
  { The~ #1~ `#2'~ is~ deprecated.~ Use~ `#3'~ instead. }

% --------------------------------------------------------------------------- %
% temporary variables
\tl_new:N  \l__modiagram_tmpa_tl
\tl_new:N  \l__modiagram_tmpb_tl
\dim_new:N \l__modiagram_tmpa_dim
\fp_new:N  \l__modiagram_tmpa_fp

% --------------------------------------------------------------------------- %
% variants of expl3 kernel commands:
\cs_generate_variant:Nn \str_case:nnF {V}
\cs_generate_variant:Nn \dim_compare:nNnTF {vNv}

% --------------------------------------------------------------------------- %
% main environment
\cs_new_protected:Npn \modiagram_start_environment:n #1
  {
    \cs_set_eq:NN \atom \__modiagram_atom:
    \cs_set_eq:NN \molecule \__modiagram_molecule:
    \cs_set_eq:NN \AO \__modiagram_arbitrary_AO:
    \cs_set_eq:NN \EnergyAxis \__modiagram_EnergyAxis:
    \dim_gzero:N \g__modiagram_min_energy_dim
    \dim_gzero:N \g__modiagram_max_energy_dim
    \int_gzero:N \g__modiagram_AO_name_int
    \keys_set:nn {modiagram/diagram} {#1}
    \__modiagram_set_labels:
    \__modiagram_tikzpicture:n { }
    \char_set_catcode_letter:N \:
    \char_set_catcode_other:N \;
  }

\cs_new_protected:Npn \modiagram_end_environment:
  {
    \bool_if:NT \l__modiagram_names_bool
      {
        \__modiagram_node:f { \tl_use:N \l__modiagram_names_style_tl }
          at
          (
            \l__modiagram_left_atom_position_dim ,
            \g__modiagram_min_energy_dim - 1cm
          )
          { \tl_use:N \l__modiagram_atom_left_name_tl } ;
        \__modiagram_node:f { \tl_use:N \l__modiagram_names_style_tl }
          at
          (
            \l__modiagram_molecule_position_dim ,
            \g__modiagram_min_energy_dim - 1cm
          )
          { \tl_use:N \l__modiagram_molecule_name_tl } ;
        \__modiagram_node:f { \tl_use:N \l__modiagram_names_style_tl }
          at
          (
            \l__modiagram_left_atom_position_dim
              + \l__modiagram_inter_atom_distance_dim ,
            \g__modiagram_min_energy_dim - 1cm
          )
          { \tl_use:N \l__modiagram_atom_right_name_tl } ;
      }
    \endtikzpicture
  }

\NewDocumentEnvironment {modiagram} {O{}}
  { \modiagram_start_environment:n {#1} }
  { \modiagram_end_environment: }

\NewDocumentEnvironment {MOdiagram} {O{}}
  {
    \msg_warning:nnnnn {modiagram} {deprecated}
      {environment}
      {MOdiagram}
      {modiagram}
    \modiagram_start_environment:n {#1}
  }
  { \modiagram_end_environment: }

\bool_new:N \l__modiagram_AO_square_bool
\bool_new:N \l__modiagram_AO_round_bool
\bool_new:N \l__modiagram_AO_circle_bool
\bool_new:N \l__modiagram_AO_fancy_bool
\bool_new:N \l__modiagram_AO_labels_bool
\bool_new:N \l__modiagram_names_bool

\tl_new:N \l__modiagram_names_style_tl
\tl_set:Nn \l__modiagram_names_style_tl {anchor=base}

\tl_new:N \l__modiagram_lines_tl
\tl_set:Nn \l__modiagram_lines_tl {dotted}

\tl_new:N \l__modiagram_orbital_label_fontsize_tl
\tl_new:N \l__modiagram_orbital_label_style_tl

\dim_new:N \l__modiagram_AO_width_dim
\dim_set:Nn \l__modiagram_AO_width_dim {10pt}

\tl_new:N \l__modiagram_AO_up_electron_tl
\tl_set:Nn \l__modiagram_AO_up_electron_tl {.4}
\tl_new:N \l__modiagram_AO_down_electron_tl
\tl_set:Nn \l__modiagram_AO_down_electron_tl {.6}

\dim_new:N \l__modiagram_left_atom_position_dim
\dim_set:Nn \l__modiagram_left_atom_position_dim {1cm}
\dim_new:N \l__modiagram_inter_atom_distance_dim
\dim_set:Nn \l__modiagram_inter_atom_distance_dim {4cm}
\dim_new:N \l__modiagram_molecule_position_dim

\dim_new:N \g__modiagram_min_energy_dim
\dim_new:N \g__modiagram_max_energy_dim

\dim_new:N \l__modiagram_energy_gain_dim
\dim_new:N \l__modiagram_energy_loss_dim

% detect lowest and highest orbital so we can draw the appropriate energy axis:
\cs_new_protected:Npn \__modiagram_energy_range:N #1
  {
    \dim_set:Nn \l__modiagram_tmpa_dim {#1}
    \dim_compare:nT { \l__modiagram_tmpa_dim < \g__modiagram_min_energy_dim }
      { \dim_gset_eq:NN \g__modiagram_min_energy_dim \l__modiagram_tmpa_dim }
    \dim_compare:nT { \l__modiagram_tmpa_dim > \g__modiagram_max_energy_dim }
      { \dim_gset_eq:NN \g__modiagram_max_energy_dim \l__modiagram_tmpa_dim }
  }

% split energy levels for creating the MO; we need to detect wether the left
% or the right AO is the lower or higher one, respectively
\cs_new_protected:Npn \__modiagram_split_energy:nNN #1#2#3
  {
    \dim_compare:vNvTF
      { l__modiagram_atom_left_#1_dim } < { l__modiagram_atom_right_#1_dim }
      {
        \dim_set_eq:Nc \l__modiagram_energy_gain_dim
          { l__modiagram_atom_left_#1_dim }
        \dim_sub:Nn \l__modiagram_energy_gain_dim {#2}
        \dim_set_eq:Nc \l__modiagram_energy_loss_dim
          { l__modiagram_atom_right_#1_dim }
        \dim_add:Nn \l__modiagram_energy_loss_dim {#3}
      }
      {
        \dim_set_eq:Nc \l__modiagram_energy_gain_dim
          { l__modiagram_atom_right_#1_dim }
        \dim_sub:Nn \l__modiagram_energy_gain_dim {#2}
        \dim_set_eq:Nc \l__modiagram_energy_loss_dim
          { l__modiagram_atom_left_#1_dim }
        \dim_add:Nn \l__modiagram_energy_loss_dim {#3}
      }
  }

% position the spin-up/spin-down el:
\cs_new_protected:Npn \__modiagram_AO_place_electron:nn #1#2
  {
    \fp_compare:nNnT {#2} < {0}
      { \msg_error:nn {modiagram} {electron-placement} }
    \fp_compare:nNnT { abs(#2) } > {1}
      { \msg_error:nn {modiagram} {electron-placement} }
    \bool_if:nTF {#1}
      { \tl_set:Nn \l__modiagram_AO_down_electron_tl {#2} }
      { \tl_set:Nn \l__modiagram_AO_up_electron_tl {#2} }
  }
\cs_generate_variant:Nn \__modiagram_AO_place_electron:nn {nx}

\cs_new_protected:Npn \__modiagram_AO_place_electrons:n #1
  {
    \fp_set:Nn \l__modiagram_tmpa_fp { .5 * ( 1 - #1 ) }
    \__modiagram_AO_place_electron:nx {0} { \fp_to_tl:N \l__modiagram_tmpa_fp }
    \fp_set:Nn \l__modiagram_tmpa_fp { .5 * ( 1 + #1 ) }
    \__modiagram_AO_place_electron:nx {1} { \fp_to_tl:N \l__modiagram_tmpa_fp }
  }

% place the right atom
\cs_new_protected:Npn \__modiagram_molecule_position:n #1
  {
    \fp_set:Nn \l__modiagram_tmpa_fp { .5 * \dim_to_fp:n {#1} }
    \dim_set:Nn \l__modiagram_molecule_position_dim
      { \fp_to_dim:N \l__modiagram_tmpa_fp + \l__modiagram_left_atom_position_dim }
  }
\cs_generate_variant:Nn \__modiagram_molecule_position:n {f}

\__modiagram_molecule_position:f
  { \dim_use:N \l__modiagram_inter_atom_distance_dim }

% set up keys for the environment
\keys_define:nn {modiagram/diagram}
  {
    style          .choice: ,
    style / plain  .code:n    =
      {
        \bool_set_false:N \l__modiagram_AO_square_bool
        \bool_set_false:N \l__modiagram_AO_round_bool
        \bool_set_false:N \l__modiagram_AO_circle_bool
        \bool_set_false:N \l__modiagram_AO_fancy_bool
        \tl_set:Nn \l__modiagram_lines_tl { dotted }
      } ,
    style / square .code:n    =
      {
        \bool_set_true:N \l__modiagram_AO_square_bool
        \bool_set_false:N \l__modiagram_AO_round_bool
        \bool_set_false:N \l__modiagram_AO_circle_bool
        \bool_set_false:N \l__modiagram_AO_fancy_bool
        \tl_set:Nn \l__modiagram_lines_tl { gray }
      } ,
    style / circle .code:n    =
      {
        \bool_set_false:N \l__modiagram_AO_square_bool
        \bool_set_false:N \l__modiagram_AO_round_bool
        \bool_set_true:N \l__modiagram_AO_circle_bool
        \bool_set_false:N \l__modiagram_AO_fancy_bool
        \tl_set:Nn \l__modiagram_lines_tl { gray }
      } ,
    style / round  .code:n    =
      {
        \bool_set_true:N \l__modiagram_AO_square_bool
        \bool_set_true:N \l__modiagram_AO_round_bool
        \bool_set_false:N \l__modiagram_AO_circle_bool
        \bool_set_false:N \l__modiagram_AO_fancy_bool
        \tl_set:Nn \l__modiagram_lines_tl { gray }
      } ,
    style / fancy  .code:n    =
      {
        \bool_set_true:N \l__modiagram_AO_square_bool
        \bool_set_true:N \l__modiagram_AO_round_bool
        \bool_set_false:N \l__modiagram_AO_circle_bool
        \bool_set_true:N \l__modiagram_AO_fancy_bool
        \tl_set:Nn \l__modiagram_lines_tl { densely~dashed }
      } ,
    lines         .tl_set:N   = \l__modiagram_lines_tl ,
    distance      .code:n     =
      {
        \dim_set:Nn \l__modiagram_inter_atom_distance_dim {#1}
        \__modiagram_molecule_position:n {#1}
      } ,
    AO-width      .tl_set:N   = \l__modiagram_AO_width_dim ,
    up-el-pos     .code:n     = \__modiagram_AO_place_electron:nn {0} {#1} ,
    down-el-pos   .code:n     = \__modiagram_AO_place_electron:nn {1} {#1} ,
    el-sep        .code:n     = \__modiagram_AO_place_electrons:n {#1} ,
    labels        .bool_set:N = \l__modiagram_AO_labels_bool ,
    names         .bool_set:N = \l__modiagram_names_bool ,
    names-style   .tl_set:N   = \l__modiagram_names_style_tl ,
    names-style-add .code:n   =
      { \tl_put_right:Nn \l__modiagram_names_style_tl { , #1 } } ,
    labels-fs     .tl_set:N   = \l__modiagram_orbital_label_fontsize_tl ,
    labels-style  .tl_set:N   = \l__modiagram_orbital_label_style_tl
  }

\ProcessKeysOptions { modiagram / diagram }

% --------------------------------------------------------------------------- %
% setup command
\NewDocumentCommand \setmodiagram {+m}
  { \keys_set:nn {modiagram/diagram} {#1} }

\NewDocumentCommand \MOsetup {+m}
  {
    \msg_warning:nnnnn {modiagram} {deprecated}
      {command}
      {\MOsetup}
      {\setmodiagram}
    \keys_set:nn {modiagram/diagram} {#1}
  }

% --------------------------------------------------------------------------- %
% draw AOs
\tl_new:N \l__modiagram_atom_left_name_tl
\tl_new:N \l__modiagram_atom_right_name_tl

\bool_new:N \l__modiagram_left_atom_placed_bool
\bool_set_false:N \l__modiagram_left_atom_placed_bool
\bool_new:N \l__modiagram_right_atom_placed_bool
\bool_set_false:N \l__modiagram_right_atom_placed_bool

\NewDocumentCommand \__modiagram_atom: {omm}
  {
    \__modiagram_atom_disable:
    \keys_set:nn {modiagram/atom} { position = #2 }
    \keys_set:nn {modiagram/atom} {#3}
    \bool_if:NTF \l__modiagram_atom_left_bool
      { \bool_set_true:N \l__modiagram_left_atom_placed_bool }
      { \bool_set_true:N \l__modiagram_right_atom_placed_bool }
    \IfNoValueF {#1}
      {
        \bool_if:NTF \l__modiagram_atom_left_bool
          { \tl_set:Nn \l__modiagram_atom_left_name_tl {#1} }
          { \tl_set:Nn \l__modiagram_atom_right_name_tl {#1} }
      }
    \__modiagram_one_s:
    \__modiagram_two_s:
    \__modiagram_two_p:
%     \__modiagram_three_s:
%     \__modiagram_three_p:
%     \__modiagram_four_s:
  }

\bool_new:N \l__modiagram_atom_left_bool

% is \atom placed left or right?
\keys_define:nn {modiagram/atom}
  {
    position         .choice: ,
    position / left  .code:n =
      { \bool_set_true:N \l__modiagram_atom_left_bool } ,
    position / right .code:n =
      { \bool_set_false:N \l__modiagram_atom_left_bool }
  }

\dim_new:N \l__modiagram_atom_xshift_dim

% these are going to be our `arguments' for the \atom command:
\keys_define:nn {modiagram/atom}
  {
    1s          .code:n    = { \__modiagram_one_s_set:n {#1} } ,
    1s          .default:n = { 0 ; pair } ,
    2s          .code:n    = { \__modiagram_two_s_set:n {#1} } ,
    2s          .default:n = { 2 ; pair } ,
    2p          .code:n    = { \__modiagram_two_p_set:n {#1} } ,
    2p          .default:n = { 5 ; pair , pair , pair } ,
    color       .code:n    = { \__modiagram_electron_color_set:n {#1} } ,
    label       .code:n    = { \__modiagram_orbital_label_set:n {#1} } ,
    up-el-pos   .code:n    = { \__modiagram_electron_up_set:n {#1} } ,
    down-el-pos .code:n    = { \__modiagram_electron_down_set:n {#1} }
  }

% --------------------------------------------------------------------------- %
% 1s orbital
\dim_new:N \l__modiagram_atom_one_s_dim
\dim_new:N \l__modiagram_atom_left_one_s_dim
\dim_new:N \l__modiagram_atom_right_one_s_dim

\tl_new:N \l__modiagram_AO_one_s_electron_tl

\bool_new:N \l__modiagram_atom_one_s_bool
\bool_new:N \l__modiagram_one_s_left_bool
\bool_set_false:N \l__modiagram_one_s_left_bool
\bool_new:N \l__modiagram_one_s_right_bool
\bool_set_false:N \l__modiagram_one_s_right_bool

% read 1s argument
\cs_new_protected:Npn \__modiagram_one_s_set:n #1
  {
    \bool_set_true:N \l__modiagram_atom_one_s_bool
    \tl_if_in:onTF {#1} {;}
      { \__modiagram_one_s_set_aux:n #1 \q_stop }
      {
        \tl_clear:N \l__modiagram_AO_one_s_electron_tl
        \tl_if_blank:nTF {#1}
          { \dim_set:Nn \l__modiagram_atom_one_s_dim {0cm} }
          { \dim_set:Nn \l__modiagram_atom_one_s_dim {#1cm} }
      }
  }

\cs_new_protected:Npn \__modiagram_one_s_set_aux:n #1;#2 \q_stop
  {
    \tl_if_blank:nTF {#1}
      { \dim_set:Nn \l__modiagram_atom_one_s_dim {0cm} }
      { \dim_set:Nn \l__modiagram_atom_one_s_dim {#1cm} }
    \tl_set:Nn \l__modiagram_AO_one_s_electron_tl {#2}
  }

% display 1s orbital
\cs_new_protected:Npn \__modiagram_one_s:
  {
    \bool_if:NT \l__modiagram_atom_one_s_bool
      {
        \bool_if:NTF \l__modiagram_atom_left_bool
          {
            \dim_set_eq:NN
              \l__modiagram_atom_xshift_dim
              \l__modiagram_left_atom_position_dim
            \dim_set_eq:NN
              \l__modiagram_atom_left_one_s_dim
              \l__modiagram_atom_one_s_dim
            \bool_set_true:N \l__modiagram_one_s_left_bool
          }
          {
            \dim_set:Nn \l__modiagram_atom_xshift_dim
              {
                \l__modiagram_left_atom_position_dim
                  + \l__modiagram_inter_atom_distance_dim
              }
            \dim_set_eq:NN
              \l__modiagram_atom_right_one_s_dim
              \l__modiagram_atom_one_s_dim
            \bool_set_true:N \l__modiagram_one_s_right_bool
          }
        \__modiagram_energy_range:N \l__modiagram_atom_one_s_dim
        \__modiagram_node:n { inner~sep=0 , outer~sep=0 }
          (1s
            \bool_if:NTF \l__modiagram_atom_left_bool
              {left}
              {right}
          )
          at ( \l__modiagram_atom_xshift_dim , \l__modiagram_atom_one_s_dim )
          {
            \bool_if:NTF \l__modiagram_atom_left_bool
              {
                \__modiagram_get_electron_color:n {1sleft}
                \__modiagram_get_electron_up:n {1sleft}
                \__modiagram_get_electron_down:n {1sleft}
              }
              {
                \__modiagram_get_electron_color:n {1sright}
                \__modiagram_get_electron_up:n {1sright}
                \__modiagram_get_electron_down:n {1sright}
              }
            \__modiagram_AO:ff
              { \tl_use:N \l__modiagram_AO_one_s_electron_tl }
              { \tl_use:N \l__modiagram_electron_current_color_tl }
          } ;
        \bool_if:NTF \l__modiagram_atom_left_bool
          { \__modiagram_get_orbital_label:n {1sleft} }
          { \__modiagram_get_orbital_label:n {1sright} }
      }
  }

% --------------------------------------------------------------------------- %
% 2s orbital
\dim_new:N \l__modiagram_atom_two_s_dim
\dim_new:N \l__modiagram_atom_left_two_s_dim
\dim_new:N \l__modiagram_atom_right_two_s_dim

\tl_new:N \l__modiagram_AO_two_s_electron_tl

\bool_new:N \l__modiagram_atom_two_s_bool
\bool_new:N \l__modiagram_two_s_left_bool
\bool_set_false:N \l__modiagram_two_s_left_bool
\bool_new:N \l__modiagram_two_s_right_bool
\bool_set_false:N \l__modiagram_two_s_right_bool

% read 2s argument
\cs_new_protected:Npn \__modiagram_two_s_set:n #1
  {
    \bool_set_true:N \l__modiagram_atom_two_s_bool
    \tl_if_in:onTF {#1} {;}
      { \__modiagram_two_s_set_aux:n #1 \q_stop }
      {
        \tl_clear:N \l__modiagram_AO_two_s_electron_tl
        \tl_if_blank:nTF {#1}
          { \dim_set:Nn \l__modiagram_atom_two_s_dim {2cm} }
          { \dim_set:Nn \l__modiagram_atom_two_s_dim {#1cm} }
      }
  }

\cs_new_protected:Npn \__modiagram_two_s_set_aux:n #1;#2 \q_stop
  {
    \tl_if_blank:nTF {#1}
      { \dim_set:Nn \l__modiagram_atom_two_s_dim {2cm} }
      { \dim_set:Nn \l__modiagram_atom_two_s_dim {#1cm} }
    \tl_set:Nn \l__modiagram_AO_two_s_electron_tl {#2}
  }

% display 2s orbital
\cs_new_protected:Npn \__modiagram_two_s:
  {
    \bool_if:NT \l__modiagram_atom_two_s_bool
      {
        \bool_if:NTF \l__modiagram_atom_left_bool
          {
            \dim_set_eq:NN
              \l__modiagram_atom_xshift_dim
              \l__modiagram_left_atom_position_dim
            \dim_set_eq:NN
              \l__modiagram_atom_left_two_s_dim
              \l__modiagram_atom_two_s_dim
            \bool_set_true:N \l__modiagram_two_s_left_bool
          }
          {
            \dim_set:Nn \l__modiagram_atom_xshift_dim
              {
                \l__modiagram_left_atom_position_dim
                  + \l__modiagram_inter_atom_distance_dim
              }
            \dim_set_eq:NN
              \l__modiagram_atom_right_two_s_dim
              \l__modiagram_atom_two_s_dim
            \bool_set_true:N \l__modiagram_two_s_right_bool
          }
        \__modiagram_energy_range:N \l__modiagram_atom_two_s_dim
        \__modiagram_node:n { inner~sep=0 , outer~sep=0 }
          (2s
            \bool_if:NTF \l__modiagram_atom_left_bool
              {left}
              {right}
          )
          at ( \l__modiagram_atom_xshift_dim , \l__modiagram_atom_two_s_dim )
          {
            \bool_if:NTF \l__modiagram_atom_left_bool
              {
                \__modiagram_get_electron_color:n {2sleft}
                \__modiagram_get_electron_up:n {2sleft}
                \__modiagram_get_electron_down:n {2sleft}
              }
              {
                \__modiagram_get_electron_color:n {2sright}
                \__modiagram_get_electron_up:n {2sright}
                \__modiagram_get_electron_down:n {2sright}
              }
            \__modiagram_AO:ff
              { \tl_use:N \l__modiagram_AO_two_s_electron_tl }
              { \tl_use:N \l__modiagram_electron_current_color_tl }
          } ;
        \bool_if:NTF \l__modiagram_atom_left_bool
          { \__modiagram_get_orbital_label:n {2sleft} }
          { \__modiagram_get_orbital_label:n {2sright} }
      }
  }

% --------------------------------------------------------------------------- %
% 2p orbital
\dim_new:N \l__modiagram_atom_two_p_dim
\dim_new:N \l__modiagram_atom_left_two_p_dim
\dim_new:N \l__modiagram_atom_right_two_p_dim

\dim_new:N \l__modiagram_atom_px_shift_dim
\dim_new:N \l__modiagram_atom_py_shift_dim
\dim_new:N \l__modiagram_atom_pz_shift_dim

\tl_new:N \l__modiagram_AO_two_px_electron_tl
\tl_new:N \l__modiagram_AO_two_py_electron_tl
\tl_new:N \l__modiagram_AO_two_pz_electron_tl

\bool_new:N \l__modiagram_atom_two_p_bool
\bool_new:N \l__modiagram_two_p_left_bool
\bool_set_false:N \l__modiagram_two_p_left_bool
\bool_new:N \l__modiagram_two_p_right_bool
\bool_set_false:N \l__modiagram_two_p_right_bool

% read 2p argument
\cs_new_protected:Npn \__modiagram_two_p_set:n #1
  {
    \bool_set_true:N \l__modiagram_atom_two_p_bool
    \tl_if_in:onTF {#1} {;}
      { \__modiagram_two_p_set_aux_i:n #1 \q_stop }
      {
        \tl_clear:N \l__modiagram_AO_two_px_electron_tl
        \tl_clear:N \l__modiagram_AO_two_py_electron_tl
        \tl_clear:N \l__modiagram_AO_two_pz_electron_tl
        \tl_if_blank:nTF {#1}
          { \dim_set:Nn \l__modiagram_atom_two_p_dim { 5 cm } }
          { \dim_set:Nn \l__modiagram_atom_two_p_dim { #1 cm } }
      }
  }

\cs_new_protected:Npn \__modiagram_two_p_set_aux_i:n #1;#2 \q_stop
  {
    \tl_if_blank:nTF {#1}
      { \dim_set:Nn \l__modiagram_atom_two_p_dim { 5 cm } }
      { \dim_set:Nn \l__modiagram_atom_two_p_dim { #1 cm } }
    \__modiagram_two_p_set_aux_ii:n {#2}
  }

\NewDocumentCommand \__modiagram_two_p_set_aux_ii:n
  { > { \SplitArgument {2} {,} } m }
  { \__modiagram_two_p_set_aux_iii:nnn #1 }

\cs_set_nopar:Npn \__modiagram_two_p_set_aux_iii:nnn #1#2#3
  {
    \tl_if_blank:nF {#1} { \tl_set:Nn \l__modiagram_AO_two_px_electron_tl {#1} }
    \tl_if_blank:nF {#2} { \tl_set:Nn \l__modiagram_AO_two_py_electron_tl {#2} }
    \tl_if_blank:nF {#3} { \tl_set:Nn \l__modiagram_AO_two_pz_electron_tl {#3} }
  }

% place the x, y, and z orbitals
\cs_new_protected:Npn \__modiagram_p_orbitals_shift:nnn #1#2#3
  {
    \fp_set:Nn \l__modiagram_tmpa_fp { \dim_to_fp:n { \l__modiagram_AO_width_dim } }
    {
      \fp_set:Nn \l__modiagram_tmpa_fp { #1 * \l__modiagram_tmpa_fp }
      \dim_gset:Nn \l__modiagram_atom_px_shift_dim { \fp_to_dim:N \l__modiagram_tmpa_fp }
    }
    {
      \fp_set:Nn \l__modiagram_tmpa_fp { #2 * \l__modiagram_tmpa_fp }
      \dim_gset:Nn \l__modiagram_atom_py_shift_dim { \fp_to_dim:N \l__modiagram_tmpa_fp }
    }
    {
      \fp_set:Nn \l__modiagram_tmpa_fp { #3 * \l__modiagram_tmpa_fp }
      \dim_gset:Nn \l__modiagram_atom_pz_shift_dim { \fp_to_dim:N \l__modiagram_tmpa_fp }
    }
  }

% display 2p orbitals
\cs_new_protected:Npn \__modiagram_two_p:
  {
    \bool_if:NT \l__modiagram_atom_two_p_bool
      {
        \bool_if:NTF \l__modiagram_atom_left_bool
          {
            \dim_set_eq:NN
              \l__modiagram_atom_xshift_dim
              \l__modiagram_left_atom_position_dim 
            \__modiagram_p_orbitals_shift:nnn {-4} {-2} {0}
            \dim_set_eq:NN
              \l__modiagram_atom_left_two_p_dim
              \l__modiagram_atom_two_p_dim
            \dim_set_eq:NN
              \l_energy_axis_dim
              \l__modiagram_atom_px_shift_dim
            \bool_set_true:N \l__modiagram_two_p_left_bool
          }
          {
            \dim_set:Nn \l__modiagram_atom_xshift_dim
              {
                \l__modiagram_left_atom_position_dim
                  + \l__modiagram_inter_atom_distance_dim
              }
            \__modiagram_p_orbitals_shift:nnn {0} {2} {4}
            \dim_set_eq:NN
              \l__modiagram_atom_right_two_p_dim
              \l__modiagram_atom_two_p_dim
            \bool_set_true:N \l__modiagram_two_p_right_bool
          }
        \__modiagram_energy_range:N \l__modiagram_atom_two_p_dim
        \__modiagram_node:n { inner~sep=0 , outer~sep=0 }
          (2pz
            \bool_if:NTF \l__modiagram_atom_left_bool
              {left}
              {right}
          )
          at
          (
            \l__modiagram_atom_xshift_dim + \l__modiagram_atom_pz_shift_dim ,
            \l__modiagram_atom_two_p_dim
          )
          {
            \bool_if:NTF \l__modiagram_atom_left_bool
              {
                \__modiagram_get_electron_color:n {2pzleft}
                \__modiagram_get_electron_up:n {2pzleft}
                \__modiagram_get_electron_down:n {2pzleft}
              }
              {
                \__modiagram_get_electron_color:n {2pzright}
                \__modiagram_get_electron_up:n {2pzright}
                \__modiagram_get_electron_down:n {2pzright}
              }
            \__modiagram_AO:ff
              { \tl_use:N \l__modiagram_AO_two_pz_electron_tl }
              { \tl_use:N \l__modiagram_electron_current_color_tl }
          } ;
        \bool_if:NTF \l__modiagram_atom_left_bool
          { \__modiagram_get_orbital_label:n {2pzleft} }
          { \__modiagram_get_orbital_label:n {2pzright} }
        \__modiagram_node:n { inner~sep=0 , outer~sep=0 }
          (2py
            \bool_if:NTF \l__modiagram_atom_left_bool
              {left}
              {right}
          )
          at
          (
            \l__modiagram_atom_xshift_dim + \l__modiagram_atom_py_shift_dim ,
            \l__modiagram_atom_two_p_dim
          )
          {
            \bool_if:NTF \l__modiagram_atom_left_bool
              {
                \__modiagram_get_electron_color:n {2pyleft}
                \__modiagram_get_electron_up:n {2pyleft}
                \__modiagram_get_electron_down:n {2pyleft}
              }
              {
                \__modiagram_get_electron_color:n {2pyright}
                \__modiagram_get_electron_up:n {2pyright}
                \__modiagram_get_electron_down:n {2pyright}
              }
            \__modiagram_AO:ff
              { \tl_use:N \l__modiagram_AO_two_py_electron_tl }
              { \tl_use:N \l__modiagram_electron_current_color_tl }
          } ;
        \bool_if:NTF \l__modiagram_atom_left_bool
          { \__modiagram_get_orbital_label:n {2pyleft} }
          { \__modiagram_get_orbital_label:n {2pyright} }
        \__modiagram_node:n { inner~sep=0 , outer~sep=0 }
          (2px
            \bool_if:NTF \l__modiagram_atom_left_bool
              {left}
              {right}
          )
          at
          (
            \l__modiagram_atom_xshift_dim + \l__modiagram_atom_px_shift_dim ,
            \l__modiagram_atom_two_p_dim
          )
          {
            \bool_if:NTF \l__modiagram_atom_left_bool
              {
                \__modiagram_get_electron_color:n {2pxleft}
                \__modiagram_get_electron_up:n {2pxleft}
                \__modiagram_get_electron_down:n {2pxleft}
              }
              {
                \__modiagram_get_electron_color:n {2pxright}
                \__modiagram_get_electron_up:n {2pxright}
                \__modiagram_get_electron_down:n {2pxright}
              }
            \__modiagram_AO:ff
              { \tl_use:N \l__modiagram_AO_two_px_electron_tl }
              { \tl_use:N \l__modiagram_electron_current_color_tl }
          } ;
        \bool_if:NTF \l__modiagram_atom_left_bool
          { \__modiagram_get_orbital_label:n {2pxleft} }
          { \__modiagram_get_orbital_label:n {2pxright} }
      }  
  }

% --------------------------------------------------------------------------- %
% draw MOs
\tl_new:N \l__modiagram_molecule_name_tl

\NewDocumentCommand \__modiagram_molecule: {om}
  {
    \bool_if:nTF
      {
        \l__modiagram_left_atom_placed_bool
          &&
        \l__modiagram_right_atom_placed_bool
      }
      {
        \__modiagram_molecule_disable:
        \keys_set:nn {modiagram/molecule} {#2}
        \IfNoValueF {#1}
          { \tl_set:Nn \l__modiagram_molecule_name_tl {#1} }
        \__modiagram_one_mo_s:
        \__modiagram_two_mo_s:
        \__modiagram_two_mo_p:
%         \__modiagram_three_mo_s:
%         \__modiagram_three_mo_p:
%         \__modiagram_four_mo_s:
      }
      { \msg_error:nn {modiagram} {missing-atom} }
  }

% these are going to be our molecule arguments:
\keys_define:nn {modiagram/molecule}
  {
    1sMO        .code:n    = { \__modiagram_one_mo_s_set:n {#1} } ,
    1sMO        .default:n = { .5 ; pair , pair } ,
    2sMO        .code:n    = { \__modiagram_two_mo_s_set:n {#1} } ,
    2sMO        .default:n = { .5 ; pair , pair } ,
    2pMO        .code:n    = { \__modiagram_two_mo_p_set:n {#1} } ,
    2pMO        .default:n = { 1.5 , .5 ; pair , pair , pair , pair , pair , pair } ,
    color       .code:n    = { \__modiagram_electron_color_set:n {#1} } ,
    label       .code:n    = { \__modiagram_orbital_label_set:n {#1} } ,
    up-el-pos   .code:n    = { \__modiagram_electron_up_set:n {#1} } ,
    down-el-pos .code:n    = { \__modiagram_electron_down_set:n {#1} }
  }

% --------------------------------------------------------------------------- %
% 1sigma orbitals
\dim_new:N \l__modiagram_molecule_one_sigma_dim
\dim_new:N \l__modiagram_molecule_one_sigmas_dim

\tl_new:N \l__modiagram_MO_one_sigma_s_electron_tl
\tl_new:N \l__modiagram_MO_one_sigmas_s_electron_tl
\bool_new:N \l__modiagram_molecule_one_mo_s_bool

% read 1sMO argument
\cs_new_protected:Npn \__modiagram_one_mo_s_set:n #1
  {
    \bool_set_true:N \l__modiagram_molecule_one_mo_s_bool
    \tl_if_in:onTF {#1} { ; }
      { \__modiagram_one_mo_s_set_aux:n #1 \q_stop }
      { \__modiagram_one_mo_s_set_energy:n {#1} }
  }

\cs_new_protected:Npn \__modiagram_one_mo_s_set_aux:n #1;#2 \q_stop
  {
    \__modiagram_one_mo_s_set_energy:n {#1}
    \__modiagram_one_mo_s_set_electrons:n {#2}
  }

\cs_new_protected:Npn \__modiagram_one_mo_s_set_energy:n #1
  {
    \tl_if_in:onTF {#1} {/}
      { \__modiagram_one_mo_s_set_energy_aux:n #1 \q_stop }
      {
        \tl_if_blank:nTF {#1}
          {
            \dim_set:Nn \l__modiagram_molecule_one_sigma_dim {.5cm}
            \dim_set:Nn \l__modiagram_molecule_one_sigmas_dim {.5cm}
          }
          {
            \dim_set:Nn \l__modiagram_molecule_one_sigma_dim {#1cm}
            \dim_set:Nn \l__modiagram_molecule_one_sigmas_dim {#1cm}
          }
      }
  }

\cs_new_protected:Npn \__modiagram_one_mo_s_set_energy_aux:n #1/#2 \q_stop
  {
    \tl_if_blank:nTF {#1}
      { \dim_set:Nn \l__modiagram_molecule_one_sigma_dim {.5cm} }
      { \dim_set:Nn \l__modiagram_molecule_one_sigma_dim {#1cm} }
    \tl_if_blank:nTF {#2}
      { \dim_set:Nn \l__modiagram_molecule_one_sigmas_dim {.5cm} }
      { \dim_set:Nn \l__modiagram_molecule_one_sigmas_dim {#2cm} }
  }

\cs_set_nopar:Npn \__modiagram_one_mo_s_set_electrons:n #1
  {
    \tl_if_in:onTF {#1} {,}
      { \__modiagram_one_mo_s_set_electrons_aux:n #1 \q_stop }
      { \tl_set:Nn \l__modiagram_MO_one_sigma_s_electron_tl {#1} }
  }

\cs_new_protected:Npn \__modiagram_one_mo_s_set_electrons_aux:n #1,#2 \q_stop
  {
    \tl_set:Nn \l__modiagram_MO_one_sigma_s_electron_tl {#1}
    \tl_set:Nn \l__modiagram_MO_one_sigmas_s_electron_tl {#2}
  }

% display 1sigma/sigma* MOs:
\cs_new_protected:Npn \__modiagram_one_mo_s:
  {
    \bool_if:NT \l__modiagram_molecule_one_mo_s_bool
      {
        \bool_if:nF
          { \l__modiagram_one_s_left_bool && \l__modiagram_one_s_right_bool }
          { \msg_error:nnnn {modiagram} {missing-AO} {1sMO} {1s} }
        \__modiagram_split_energy:nNN {one_s}
          \l__modiagram_molecule_one_sigma_dim
          \l__modiagram_molecule_one_sigmas_dim
        \__modiagram_energy_range:N \l__modiagram_energy_gain_dim
        \__modiagram_energy_range:N \l__modiagram_energy_loss_dim
        \__modiagram_node:n { inner~sep=0 , outer~sep=0 }
          (1sigma)
          at
          (
            \l__modiagram_molecule_position_dim ,
            \l__modiagram_energy_gain_dim
          )
          {
            \__modiagram_get_electron_color:n {1sigma}
            \__modiagram_get_electron_up:n {1sigma}
            \__modiagram_get_electron_down:n {1sigma}
            \__modiagram_AO:ff
              { \tl_use:N \l__modiagram_MO_one_sigma_s_electron_tl }
              { \tl_use:N \l__modiagram_electron_current_color_tl }
          } ;
        \__modiagram_get_orbital_label:n { 1sigma }
        \__modiagram_node:n { inner~sep=0 , outer~sep=0 }
          (1sigma*)
          at
          (
            \l__modiagram_molecule_position_dim ,
            \l__modiagram_energy_loss_dim
          )
          {
            \__modiagram_get_electron_color:n {1sigma*}
            \__modiagram_get_electron_up:n {1sigma*}
            \__modiagram_get_electron_down:n {1sigma*}
            \__modiagram_AO:ff
              { \tl_use:N \l__modiagram_MO_one_sigmas_s_electron_tl }
              { \tl_use:N \l__modiagram_electron_current_color_tl }
          } ;
        \__modiagram_get_orbital_label:n {1sigma*}
        \__modiagram_draw:f
          { \l__modiagram_lines_tl }
          (1s left.east)  -- (1sigma.west)  (1sigma.east)   -- (1s right.west)
          (1s left.east)  -- (1sigma*.west) (1sigma*.east)  -- (1s right.west) ;
      }
  }

% --------------------------------------------------------------------------- %
% 2sigma orbitals
\dim_new:N \l__modiagram_molecule_two_sigma_dim
\dim_new:N \l__modiagram_molecule_two_sigmas_dim

\tl_new:N \l__modiagram_MO_two_sigma_s_electron_tl
\tl_new:N \l__modiagram_MO_two_sigmas_s_electron_tl
\bool_new:N \l__modiagram_molecule_two_mo_s_bool

% read 2sMO argument
\cs_new_protected:Npn \__modiagram_two_mo_s_set:n #1
  {
    \bool_set_true:N \l__modiagram_molecule_two_mo_s_bool
    \tl_if_in:onTF {#1} {;}
      { \__modiagram_two_mo_s_set_aux:n #1 \q_stop }
      { \__modiagram_two_mo_s_set_energy:n {#1} }
  }

\cs_new_protected:Npn \__modiagram_two_mo_s_set_aux:n #1;#2 \q_stop
  {
    \__modiagram_two_mo_s_set_energy:n {#1}
    \__modiagram_two_mo_s_set_electrons:n {#2}
  }

\cs_new_protected:Npn \__modiagram_two_mo_s_set_energy:n #1
  {
    \tl_if_in:onTF {#1} {/}
      { \__modiagram_two_mo_s_set_energy_aux:n #1 \q_stop }
      {
        \tl_if_blank:nTF {#1}
          {
            \dim_set:Nn \l__modiagram_molecule_two_sigma_dim {.5cm}
            \dim_set:Nn \l__modiagram_molecule_two_sigmas_dim {.5cm}
          }
          {
            \dim_set:Nn \l__modiagram_molecule_two_sigma_dim {#1cm}
            \dim_set:Nn \l__modiagram_molecule_two_sigmas_dim {#1cm}
          }
      }
  }

\cs_new_protected:Npn \__modiagram_two_mo_s_set_energy_aux:n #1/#2 \q_stop
  {
    \tl_if_blank:nTF {#1}
      { \dim_set:Nn \l__modiagram_molecule_two_sigma_dim {.5cm} }
      { \dim_set:Nn \l__modiagram_molecule_two_sigma_dim {#1cm} }
    \tl_if_blank:nTF {#2}
      { \dim_set:Nn \l__modiagram_molecule_two_sigmas_dim {.5cm} }
      { \dim_set:Nn \l__modiagram_molecule_two_sigmas_dim {#2cm} }
  }

\cs_set_nopar:Npn \__modiagram_two_mo_s_set_electrons:n #1
  {
    \tl_if_in:onTF {#1} {,}
      { \__modiagram_two_mo_s_set_electrons_aux:n #1 \q_stop }
      { \tl_set:Nn \l__modiagram_MO_two_sigma_s_electron_tl {#1} }
  }

\cs_new_protected:Npn \__modiagram_two_mo_s_set_electrons_aux:n #1,#2 \q_stop
  {
    \tl_set:Nn \l__modiagram_MO_two_sigma_s_electron_tl {#1}
    \tl_set:Nn \l__modiagram_MO_two_sigmas_s_electron_tl {#2}
  }

% display 2sigma/sigma* MOs:
\cs_new_protected:Npn \__modiagram_two_mo_s:
  {
    \bool_if:NT \l__modiagram_molecule_two_mo_s_bool
      {
        \bool_if:nF
          { \l__modiagram_two_s_left_bool && \l__modiagram_two_s_right_bool }
          { \msg_error:nnnn {modiagram} {missing-AO} {2sMO} {2s} }
        \__modiagram_split_energy:nNN {two_s}
          \l__modiagram_molecule_two_sigma_dim
          \l__modiagram_molecule_two_sigmas_dim
        \__modiagram_energy_range:N \l__modiagram_energy_gain_dim
        \__modiagram_energy_range:N \l__modiagram_energy_loss_dim
        \__modiagram_node:n { inner~sep=0 , outer~sep=0 }
          (2sigma)
          at
          (
            \l__modiagram_molecule_position_dim ,
            \l__modiagram_energy_gain_dim
          )
          {
            \__modiagram_get_electron_color:n {2sigma}
            \__modiagram_get_electron_up:n {2sigma}
            \__modiagram_get_electron_down:n {2sigma}
            \__modiagram_AO:ff
              { \tl_use:N \l__modiagram_MO_two_sigma_s_electron_tl }
              { \tl_use:N \l__modiagram_electron_current_color_tl }
          } ;
        \__modiagram_get_orbital_label:n {2sigma}
        \__modiagram_node:n { inner~sep=0 , outer~sep=0 }
          (2sigma*)
          at
          (
            \l__modiagram_molecule_position_dim ,
            \l__modiagram_energy_loss_dim
          )
          {
            \__modiagram_get_electron_color:n {2sigma*}
            \__modiagram_get_electron_up:n {2sigma*}
            \__modiagram_get_electron_down:n {2sigma*}
            \__modiagram_AO:ff
              { \tl_use:N \l__modiagram_MO_two_sigmas_s_electron_tl }
              { \tl_use:N \l__modiagram_electron_current_color_tl }
          } ;
        \__modiagram_get_orbital_label:n {2sigma*}
        \__modiagram_draw:f
          { \l__modiagram_lines_tl }
          (2s left.east)  -- (2sigma.west)  (2sigma.east)   -- (2s right.west)
          (2s left.east)  -- (2sigma*.west) (2sigma*.east)  -- (2s right.west) ;
      }
  }

% --------------------------------------------------------------------------- %
% 2psigma and 2ppi orbitals
\dim_new:N \l__modiagram_molecule_two_psigma_dim
\dim_new:N \l__modiagram_molecule_two_psigmas_dim
\dim_new:N \l__modiagram_molecule_two_ppi_dim
\dim_new:N \l__modiagram_molecule_two_ppis_dim

\tl_new:N \l__modiagram_MO_two_sigma_p_electron_tl
\tl_new:N \l__modiagram_MO_two_sigmas_p_electron_tl
\tl_new:N \l__modiagram_MO_two_piy_p_electron_tl
\tl_new:N \l__modiagram_MO_two_piys_p_electron_tl
\tl_new:N \l__modiagram_MO_two_piz_p_electron_tl
\tl_new:N \l__modiagram_MO_two_pizs_p_electron_tl

\bool_new:N \l__modiagram_molecule_two_mo_p_bool

% read 2pMO argument
\cs_set_nopar:Npn \__modiagram_two_mo_p_set:n #1
  {
    \bool_set_true:N \l__modiagram_molecule_two_mo_p_bool
    \tl_if_in:onTF {#1} {;}
      { \__modiagram_two_mo_p_set_aux:n #1 \q_stop }
      { \__modiagram_two_mo_p_set_energy:n {#1} }
  }

\cs_set_nopar:Npn \__modiagram_two_mo_p_set_aux:n #1;#2 \q_stop
  {
    \__modiagram_two_mo_p_set_energy:n {#1}
    \__modiagram_two_mo_p_set_electrons:n {#2}
  }

\cs_new_protected:Npn \__modiagram_two_mo_p_set_energy:n #1
  {
    \tl_if_in:onTF {#1} {,}
      { \__modiagram_two_mo_p_set_energy_aux:n #1 \q_stop }
      {
        \tl_if_in:onTF {#1} {/}
          {
            \__modiagram_two_mo_p_set_energy_sigma_aux:n #1 \q_stop
            \__modiagram_two_mo_p_set_energy_pi:n {.5}
          }
          {
            \__modiagram_two_mo_p_set_energy_sigma:n {#1}
            \__modiagram_two_mo_p_set_energy_pi:n {.5}
          }
      }
  }

\cs_new_protected:Npn \__modiagram_two_mo_p_set_energy_aux:n #1,#2 \q_stop
  {
    \tl_if_in:onTF {#1} {/}
      { \__modiagram_two_mo_p_set_energy_sigma_aux:n #1 \q_stop }
      { \__modiagram_two_mo_p_set_energy_sigma:n {#1} }
    \tl_if_in:onTF {#2} {/}
      { \__modiagram_two_mo_p_set_energy_pi_aux:n #2 \q_stop }
      { \__modiagram_two_mo_p_set_energy_pi:n {#2} }
  }

\cs_new_protected:Npn \__modiagram_two_mo_p_set_energy_sigma:n #1
  {
    \tl_if_blank:nTF {#1}
      {
        \dim_set:Nn \l__modiagram_molecule_two_psigma_dim {1.5cm}
        \dim_set:Nn \l__modiagram_molecule_two_psigmas_dim {1.5cm}
      }
      {
        \dim_set:Nn \l__modiagram_molecule_two_psigma_dim {#1cm}
        \dim_set:Nn \l__modiagram_molecule_two_psigmas_dim {#1cm}
      }
  }

\cs_new_protected:Npn \__modiagram_two_mo_p_set_energy_sigma_aux:n #1/#2 \q_stop
  {
    \tl_if_blank:nTF {#1}
      { \dim_set:Nn \l__modiagram_molecule_two_psigma_dim {1.5cm} }
      { \dim_set:Nn \l__modiagram_molecule_two_psigma_dim {#1cm} }
    \tl_if_blank:nTF {#2}
      { \dim_set:Nn \l__modiagram_molecule_two_psigmas_dim {1.5cm} }
      { \dim_set:Nn \l__modiagram_molecule_two_psigmas_dim {#2cm} }
  }

\cs_new_protected:Npn \__modiagram_two_mo_p_set_energy_pi:n #1
  {
    \tl_if_blank:nTF {#1}
      {
        \dim_set:Nn \l__modiagram_molecule_two_ppi_dim {.5cm}
        \dim_set:Nn \l__modiagram_molecule_two_ppis_dim {.5cm}
      }
      {
        \dim_set:Nn \l__modiagram_molecule_two_ppi_dim {#1cm}
        \dim_set:Nn \l__modiagram_molecule_two_ppis_dim {#1cm}
      }
  }

\cs_new_protected:Npn \__modiagram_two_mo_p_set_energy_pi_aux:n #1/#2 \q_stop
  {
    \tl_if_blank:nTF {#1}
      { \dim_set:Nn \l__modiagram_molecule_two_ppi_dim {.5cm} }
      { \dim_set:Nn \l__modiagram_molecule_two_ppi_dim {#1cm} }
    \tl_if_blank:nTF {#2}
      { \dim_set:Nn \l__modiagram_molecule_two_ppis_dim {.5cm} }
      { \dim_set:Nn \l__modiagram_molecule_two_ppis_dim {#2cm} }
  }

\NewDocumentCommand \__modiagram_two_mo_p_set_electrons:n
  { > { \SplitArgument {5} {,} } m }
  { \__modiagram_two_mo_p_set_electrons_aux:n #1 }

\cs_new_protected:Npn \__modiagram_two_mo_p_set_electrons_aux:n #1#2#3#4#5#6
  {
    \tl_set:Nn \l__modiagram_MO_two_sigma_p_electron_tl {#1}
    \tl_set:Nn \l__modiagram_MO_two_sigmas_p_electron_tl {#6}
    \tl_set:Nn \l__modiagram_MO_two_piy_p_electron_tl {#2}
    \tl_set:Nn \l__modiagram_MO_two_piys_p_electron_tl {#4}
    \tl_set:Nn \l__modiagram_MO_two_piz_p_electron_tl {#3}
    \tl_set:Nn \l__modiagram_MO_two_pizs_p_electron_tl {#5}
  }

% display 2sigma+pi/sigma*+pi* orbitals:
\cs_new_protected:Npn \__modiagram_two_mo_p:
  {
    \bool_if:NT \l__modiagram_molecule_two_mo_p_bool
      {
        \bool_if:nF
          { \l__modiagram_two_p_left_bool && \l__modiagram_two_p_right_bool }
          { \msg_error:nnnn {modiagram} {missing-AO} {2pMO} {2p} }
        \__modiagram_split_energy:nNN {two_p}
          \l__modiagram_molecule_two_psigma_dim
          \l__modiagram_molecule_two_psigmas_dim
        \__modiagram_energy_range:N \l__modiagram_energy_gain_dim
        \__modiagram_energy_range:N \l__modiagram_energy_loss_dim
        \__modiagram_node:n { inner~sep=0 , outer~sep=0 }
          (2psigma)
          at
          (
            \l__modiagram_molecule_position_dim ,
            \l__modiagram_energy_gain_dim
          )
          {
            \__modiagram_get_electron_color:n {2psigma}
            \__modiagram_get_electron_up:n {2psigma}
            \__modiagram_get_electron_down:n {2psigma}
            \__modiagram_AO:ff
              { \tl_use:N \l__modiagram_MO_two_sigma_p_electron_tl }
              { \tl_use:N \l__modiagram_electron_current_color_tl }
          } ;
        \__modiagram_get_orbital_label:n {2psigma}
        \__modiagram_node:n { inner~sep=0 , outer~sep=0 }
          (2psigma*)
          at
          (
            \l__modiagram_molecule_position_dim ,
            \l__modiagram_energy_loss_dim
          )
          {
            \__modiagram_get_electron_color:n {2psigma*}
            \__modiagram_get_electron_up:n {2psigma*}
            \__modiagram_get_electron_down:n {2psigma*}
            \__modiagram_AO:ff
              { \tl_use:N \l__modiagram_MO_two_sigmas_p_electron_tl }
              { \tl_use:N \l__modiagram_electron_current_color_tl }
          } ;
        \__modiagram_get_orbital_label:n {2psigma*}
        \__modiagram_split_energy:nNN {two_p}
          \l__modiagram_molecule_two_ppi_dim
          \l__modiagram_molecule_two_ppis_dim
        \__modiagram_node:n { inner~sep=0 , outer~sep=0 }
          (2piy)
          at
          (
            \l__modiagram_molecule_position_dim - \l__modiagram_AO_width_dim ,
            \l__modiagram_energy_gain_dim
          )
          {
            \__modiagram_get_electron_color:n {2piy}
            \__modiagram_get_electron_up:n {2piy}
            \__modiagram_get_electron_down:n {2piy}
            \__modiagram_AO:ff
              { \tl_use:N \l__modiagram_MO_two_piy_p_electron_tl }
              { \tl_use:N \l__modiagram_electron_current_color_tl }
          } ;
        \__modiagram_get_orbital_label:n {2piy}
        \__modiagram_node:n { inner~sep=0 , outer~sep=0 }
          (2piy*)
          at
          (
            \l__modiagram_molecule_position_dim - \l__modiagram_AO_width_dim ,
            \l__modiagram_energy_loss_dim
          )
          {
            \__modiagram_get_electron_color:n {2piy*}
            \__modiagram_get_electron_up:n {2piy*}
            \__modiagram_get_electron_down:n {2piy*}
            \__modiagram_AO:ff
              { \tl_use:N \l__modiagram_MO_two_piys_p_electron_tl }
              { \tl_use:N \l__modiagram_electron_current_color_tl }
          } ;
        \__modiagram_get_orbital_label:n {2piy*}
        \__modiagram_node:n { inner~sep=0 , outer~sep=0 }
          (2piz)
          at
          (
            \l__modiagram_molecule_position_dim + \l__modiagram_AO_width_dim ,
            \l__modiagram_energy_gain_dim
          )
          {
            \__modiagram_get_electron_color:n {2piz}
            \__modiagram_get_electron_up:n {2piz}
            \__modiagram_get_electron_down:n {2piz}
            \__modiagram_AO:ff
              { \tl_use:N \l__modiagram_MO_two_piz_p_electron_tl }
              { \tl_use:N \l__modiagram_electron_current_color_tl }
          } ;
        \__modiagram_get_orbital_label:n {2piz}
        \__modiagram_node:n { inner~sep=0 , outer~sep=0 }
          (2piz*)
          at
          (
            \l__modiagram_molecule_position_dim + \l__modiagram_AO_width_dim ,
            \l__modiagram_energy_loss_dim
          )
          {
            \__modiagram_get_electron_color:n {2piz*}
            \__modiagram_get_electron_up:n {2piz*}
            \__modiagram_get_electron_down:n {2piz*}
            \__modiagram_AO:ff
              { \tl_use:N \l__modiagram_MO_two_pizs_p_electron_tl }
              { \tl_use:N \l__modiagram_electron_current_color_tl }
          } ;
        \__modiagram_get_orbital_label:n {2piz*}
        \__modiagram_draw:f
          { \l__modiagram_lines_tl }
          (2pz left.east)  -- (2psigma.west)  (2psigma.east)   -- (2px right.west)
          (2pz left.east)  -- (2psigma*.west) (2psigma*.east)  -- (2px right.west)
          (2pz left.east)  -- (2piy.west)     (2piz.east)      -- (2px right.west)
          (2pz left.east)  -- (2piy*.west)    (2piz*.east)     -- (2px right.west) ;
      }
  }

% --------------------------------------------------------------------------- %
% draw actual AO / MO
\tl_new:N \l__modiagram_electron_color_tl

\cs_new_protected:Npn \__modiagram_AO:nn #1#2
  {
    \group_begin:
      \tl_set:Nn \l__modiagram_electron_color_tl {#2}
      \tikzpicture
        \bool_if:NT \l__modiagram_AO_fancy_bool
          {
            \draw (0,0) -- ++(-.5*\l__modiagram_AO_width_dim,0) ;
            \draw (2pt+\l__modiagram_AO_width_dim,0)
                  -- ++(.5*\l__modiagram_AO_width_dim,0) ;
          }
        \bool_if:NT \l__modiagram_AO_circle_bool
          {
            \draw (-.4142pt,0) ++(-.2071*\l__modiagram_AO_width_dim,0)
                  -- ++(-.25*\l__modiagram_AO_width_dim,0) ;
            \draw (2.4142pt,0) ++(1.2071*\l__modiagram_AO_width_dim,0)
                  -- ++(.25*\l__modiagram_AO_width_dim,0) ;
          }
        \__modiagram_node:x
          {
            inner~sep = 0 ,
            \bool_if:NT \l__modiagram_AO_square_bool {draw} ,
            \bool_if:NT \l__modiagram_AO_circle_bool {circle,draw} ,
            minimum~size =
              \bool_if:NTF \l__modiagram_AO_square_bool
                { 2pt+\l__modiagram_AO_width_dim }
                {
                  \bool_if:NTF \l__modiagram_AO_circle_bool
                    { 1.4142*(2pt+\l__modiagram_AO_width_dim) }
                    { \l__modiagram_AO_width_dim }
                } ,
            \bool_if:NT \l__modiagram_AO_round_bool
              { rounded~corners=2pt }
          }
          at (.5*\l__modiagram_AO_width_dim+1pt,0)
          {
            \tikz
              {
                \bool_if:nTF
                  { \l__modiagram_AO_square_bool || \l__modiagram_AO_circle_bool }
                  { \draw[draw=none] (0,0) -- ++(\l__modiagram_AO_width_dim,0) ; }
                  { \draw (0,0) -- ++(\l__modiagram_AO_width_dim,0) ; }
                \tl_set:Nn \l__modiagram_tmpa_tl {#1}
                \tl_remove_all:Nn \l__modiagram_tmpa_tl {~}
                \str_case:VnF \l__modiagram_tmpa_tl
                  {
                    {up}
                      {
                        \__modiagram_draw:f
                          {
                            -left~to,semithick ,
                            \l__modiagram_electron_color_tl ,
                            inner~sep=0,outer~sep=0
                          }
                          (
                            \l__modiagram_AO_up_electron_tl*\l__modiagram_AO_width_dim,
                            -.4*\l__modiagram_AO_width_dim
                          )
                          -- ++(0,.8*\l__modiagram_AO_width_dim) ;
                      }
                    {down}
                      {
                        \__modiagram_draw:f
                          { -left~to,semithick , \l__modiagram_electron_color_tl }
                          (
                            \l__modiagram_AO_down_electron_tl*\l__modiagram_AO_width_dim,
                            .4*\l__modiagram_AO_width_dim
                          )
                          -- ++(0,-.8*\l__modiagram_AO_width_dim) ;
                      }
                    {pair}
                      {
                        \__modiagram_draw:f
                          { -left~to,semithick , \l__modiagram_electron_color_tl }
                          (
                            \l__modiagram_AO_up_electron_tl*\l__modiagram_AO_width_dim,
                            -.4*\l__modiagram_AO_width_dim
                          )
                          -- ++(0,.8*\l__modiagram_AO_width_dim) ;
                        \__modiagram_draw:f
                          { -left~to,semithick , \l__modiagram_electron_color_tl }
                          (
                            \l__modiagram_AO_down_electron_tl*\l__modiagram_AO_width_dim,
                            .4*\l__modiagram_AO_width_dim
                          )
                          -- ++(0,-.8*\l__modiagram_AO_width_dim) ;
                      }
                  }
                  {}
              }
          } ;
      \endtikzpicture
    \group_end:
  }
\cs_generate_variant:Nn \__modiagram_AO:nn {ff}

% -------------------------------------------------------------------------- %
% per default disable all AOs / MOs
\cs_new_protected:Npn \__modiagram_atom_disable:
  {
    \bool_set_false:N \l__modiagram_atom_one_s_bool
    \bool_set_false:N \l__modiagram_atom_two_s_bool
    \bool_set_false:N \l__modiagram_atom_two_p_bool
%     \bool_set_false:N \l__modiagram_atom_three_s_bool
%     \bool_set_false:N \l__modiagram_atom_three_p_bool
  }

\cs_new_protected:Npn \__modiagram_molecule_disable:
  { \bool_set_false:N \l__modiagram_molecule_one_mo_s_bool }

% --------------------------------------------------------------------------- %
% arbitrary placement of AOs / MOs
\dim_new:N \l__modiagram_AO_dim % y
\dim_new:N \l__modiagram_AO_position_dim % x

\dim_new:N \l__modiagram_AO_px_shift_dim
\dim_new:N \l__modiagram_AO_py_shift_dim
\dim_new:N \l__modiagram_AO_pz_shift_dim

\tl_new:N \l__modiagram_AO_px_electron_tl
\tl_new:N \l__modiagram_AO_py_electron_tl
\tl_new:N \l__modiagram_AO_pz_electron_tl

\tl_new:N \l__modiagram_AO_x_electron_color_tl
\tl_new:N \l__modiagram_AO_y_electron_color_tl
\tl_new:N \l__modiagram_AO_z_electron_color_tl

\bool_new:N \l__modiagram_AO_label_bool
\bool_new:N \l__modiagram_AO_x_label_bool
\bool_new:N \l__modiagram_AO_y_label_bool
\bool_new:N \l__modiagram_AO_z_label_bool

\tl_new:N \l__modiagram_AO_x_label_tl
\tl_new:N \l__modiagram_AO_y_label_tl
\tl_new:N \l__modiagram_AO_z_label_tl

\bool_new:N \l__modiagram_AO_up_electron_bool
\bool_new:N \l__modiagram_AO_x_up_electron_bool
\bool_new:N \l__modiagram_AO_y_up_electron_bool
\bool_new:N \l__modiagram_AO_z_up_electron_bool
\tl_new:N \l__modiagram_AO_x_up_electron_tl
\tl_new:N \l__modiagram_AO_y_up_electron_tl
\tl_new:N \l__modiagram_AO_z_up_electron_tl

\bool_new:N \l__modiagram_AO_down_electron_bool
\bool_new:N \l__modiagram_AO_x_down_electron_bool
\bool_new:N \l__modiagram_AO_y_down_electron_bool
\bool_new:N \l__modiagram_AO_z_down_electron_bool
\tl_new:N \l__modiagram_AO_x_down_electron_tl
\tl_new:N \l__modiagram_AO_y_down_electron_tl
\tl_new:N \l__modiagram_AO_z_down_electron_tl

\tl_new:N \l__modiagram_AO_name_tl

\bool_new:N \l__modiagram_AO_p_bool
\bool_new:N \l__modiagram_AO_left_bool

\int_new:N \g__modiagram_AO_name_int

% read argument in s case:
\cs_new_protected:Npn \__modiagram_AO_s_set:n #1
  {
    \tl_if_in:onTF {#1} {;}
      { \__modiagram_AO_s_set_aux:n #1 \q_stop }
      {
        \dim_set:Nn \l__modiagram_AO_dim {#1cm}
        \tl_set:Nn \l__modiagram_AO_px_electron_tl {pair}
      }
  }

\cs_new_protected:Npn \__modiagram_AO_s_set_aux:n #1;#2 \q_stop
  {
    \dim_set:Nn \l__modiagram_AO_dim {#1cm}
    \tl_set:Nn \l__modiagram_AO_px_electron_tl {#2}
  }

% read argument in p case:
\cs_new_protected:Npn \__modiagram_AO_p_set:n #1
  {
    \tl_if_in:onTF {#1} {;}
      { \__modiagram_AO_p_set_aux:n #1 \q_stop }
      {
        \dim_set:Nn \l__modiagram_AO_dim {#1cm}
        \tl_set:Nn \l__modiagram_AO_px_electron_tl {pair}
        \tl_set:Nn \l__modiagram_AO_py_electron_tl {pair}
        \tl_set:Nn \l__modiagram_AO_pz_electron_tl {pair}
      }
  }

\cs_new_protected:Npn \__modiagram_AO_p_set_aux:n #1;#2 \q_stop
  {
    \dim_set:Nn \l__modiagram_AO_dim {#1cm}
    \__modiagram_AO_p_set_electrons:n {#2}
  }
\NewDocumentCommand \__modiagram_AO_p_set_electrons:n
  { > { \SplitArgument {2} {,} } m }
  { \__modiagram_AO_p_set_electrons_aux:n #1 }

\cs_new_protected:Npn \__modiagram_AO_p_set_electrons_aux:n #1#2#3
  {
    \tl_set:Nn \l__modiagram_AO_px_electron_tl {#1}
    \tl_set:Nn \l__modiagram_AO_py_electron_tl {#2}
    \tl_set:Nn \l__modiagram_AO_pz_electron_tl {#3}
  }

\cs_new_protected:Npn \__modiagram_AO_get_orbital_x_label:n #1
  {
    \bool_if:nT
      { \l__modiagram_AO_x_label_bool || \l__modiagram_AO_label_bool }
      {
        \__modiagram_node:x
          { below , \tl_use:N \l__modiagram_orbital_label_style_tl }
          at ( #1 . south )
          {
            \tl_use:N \l__modiagram_orbital_label_fontsize_tl
            \tl_use:N \l__modiagram_AO_x_label_tl
          } ;
      }
  }
\cs_generate_variant:Nn \__modiagram_AO_get_orbital_x_label:n {o}

\cs_new_protected:Npn \__modiagram_AO_get_orbital_y_label:n #1
  {
    \bool_if:nT
      { \l__modiagram_AO_y_label_bool || \l__modiagram_AO_label_bool }
      {
        \__modiagram_node:x
          { below , \tl_use:N \l__modiagram_orbital_label_style_tl }
          at ( #1 . south )
          {
            \tl_use:N \l__modiagram_orbital_label_fontsize_tl
            \tl_use:N \l__modiagram_AO_y_label_tl
          } ;
      }
  }
\cs_generate_variant:Nn \__modiagram_AO_get_orbital_y_label:n {o}

\cs_new_protected:Npn \__modiagram_AO_get_orbital_z_label:n #1
  {
    \bool_if:nT
      { \l__modiagram_AO_z_label_bool || \l__modiagram_AO_label_bool }
      {
        \__modiagram_node:x
          { below , \tl_use:N \l__modiagram_orbital_label_style_tl }
          at ( #1 . south )
          {
            \tl_use:N \l__modiagram_orbital_label_fontsize_tl
            \tl_use:N \l__modiagram_AO_z_label_tl
          } ;
      }
  }
\cs_generate_variant:Nn \__modiagram_AO_get_orbital_z_label:n {o}

% these are going to be our arguments
\keys_define:nn {modiagram/AO}
  {
    type     .choice: ,
    type / s .code:n = { \bool_set_false:N \l__modiagram_AO_p_bool } ,
    type / p .code:n = { \bool_set_true:N \l__modiagram_AO_p_bool }
  }

\keys_define:nn {modiagram/AO/properties}
  {
    color          .code:n   =
      {
        \tl_set:Nn \l__modiagram_AO_x_electron_color_tl {#1}
        \tl_set:Nn \l__modiagram_AO_y_electron_color_tl {#1}
        \tl_set:Nn \l__modiagram_AO_z_electron_color_tl {#1}
      } ,
    color[x]       .tl_set:N = \l__modiagram_AO_x_electron_color_tl ,
    color[y]       .tl_set:N = \l__modiagram_AO_y_electron_color_tl ,
    color[z]       .tl_set:N = \l__modiagram_AO_z_electron_color_tl ,
    label          .code:n   =
      {
        \bool_set_true:N \l__modiagram_AO_label_bool
        \tl_set:Nn \l__modiagram_AO_x_label_tl {#1}
        \tl_set:Nn \l__modiagram_AO_y_label_tl {#1}
        \tl_set:Nn \l__modiagram_AO_z_label_tl {#1}
      } ,
    label[x]       .code:n   =
      {
        \bool_set_true:N \l__modiagram_AO_x_label_bool
        \tl_set:Nn \l__modiagram_AO_x_label_tl {#1}
      } ,
    label[y]       .code:n   =
      {
        \bool_set_true:N \l__modiagram_AO_y_label_bool
        \tl_set:Nn \l__modiagram_AO_y_label_tl {#1}
      } ,
    label[z]       .code:n   =
      {
        \bool_set_true:N \l__modiagram_AO_z_label_bool
        \tl_set:Nn \l__modiagram_AO_z_label_tl {#1}
      } ,
    up-el-pos      .code:n   =
      {
        \bool_set_true:N \l__modiagram_AO_up_electron_bool
        \tl_set:Nn \l__modiagram_AO_x_up_electron_tl {#1}
        \tl_set:Nn \l__modiagram_AO_y_up_electron_tl {#1}
        \tl_set:Nn \l__modiagram_AO_z_up_electron_tl {#1}
      } ,
    up-el-pos[x]   .code:n   =
      {
        \bool_set_true:N \l__modiagram_AO_x_up_electron_bool
        \tl_set:Nn \l__modiagram_AO_x_up_electron_tl {#1}
      } ,
    up-el-pos[y]   .code:n   =
      {
        \bool_set_true:N \l__modiagram_AO_y_up_electron_bool
        \tl_set:Nn \l__modiagram_AO_y_up_electron_tl {#1}
      } ,
    up-el-pos[z]   .code:n   =
      {
        \bool_set_true:N \l__modiagram_AO_z_up_electron_bool
        \tl_set:Nn \l__modiagram_AO_y_up_electron_tl {#1}
      } ,
    down-el-pos    .code:n   =
      {
        \bool_set_true:N \l__modiagram_AO_down_electron_bool
        \tl_set:Nn \l__modiagram_AO_x_down_electron_tl {#1}
        \tl_set:Nn \l__modiagram_AO_y_down_electron_tl {#1}
        \tl_set:Nn \l__modiagram_AO_z_down_electron_tl {#1}
      } ,
    down-el-pos[x] .code:n   =
      {
        \bool_set_true:N \l__modiagram_AO_x_down_electron_bool
        \tl_set:Nn \l__modiagram_AO_x_down_electron_tl {#1}
      } ,
    down-el-pos[y] .code:n   =
      {
        \bool_set_true:N \l__modiagram_AO_y_down_electron_bool
        \tl_set:Nn \l__modiagram_AO_y_down_electron_tl {#1}
      } ,
    down-el-pos[z] .code:n   =
      {
        \bool_set_true:N \l__modiagram_AO_z_down_electron_bool
        \tl_set:Nn \l__modiagram_AO_z_down_electron_tl {#1}
      }
  }

% AO[<name>](<x-coord>){<type>}[<keyval>]{<AO-spec>}
\NewDocumentCommand \__modiagram_arbitrary_AO: {oD(){0cm}mom}
  {
    \group_begin:
    \IfNoValueTF {#1}
      {
        \int_gincr:N \g__modiagram_AO_name_int
        \tl_set:Nn \l__modiagram_AO_name_tl
          { AO \int_use:N \g__modiagram_AO_name_int }
      }
      { \tl_set:Nn \l__modiagram_AO_name_tl {#1} }
    \IfNoValueF {#4}
      { \keys_set:nn {modiagram/AO/properties} {#4} }
    \keys_set:nn {modiagram/AO} { type = #3 }
    \bool_if:NTF \l__modiagram_AO_p_bool
      { \__modiagram_AO_p_set:n {#5} }
      { \__modiagram_AO_s_set:n {#5} }
    \dim_set:Nn \l__modiagram_AO_position_dim {#2}
    \__modiagram_p_orbitals_shift:nnn {0} {2} {4}
    \__modiagram_energy_range:N \l__modiagram_AO_dim
    \__modiagram_node:n { inner~sep=0 , outer~sep=0 }
      (
        \tl_use:N \l__modiagram_AO_name_tl
        \bool_if:NT \l__modiagram_AO_p_bool {x}
      )
      at
      (
        \l__modiagram_AO_position_dim + \l__modiagram_atom_px_shift_dim ,
        \l__modiagram_AO_dim
      )
      {
        \bool_if:nT
          {
            \l__modiagram_AO_up_electron_bool ||
            \l__modiagram_AO_x_up_electron_bool
          }
          {
            \__modiagram_AO_place_electron:nx {0}
              { \tl_use:N \l__modiagram_AO_x_up_electron_tl }
          }
        \bool_if:nT
          {
            \l__modiagram_AO_down_electron_bool ||
            \l__modiagram_AO_x_down_electron_bool
          }
          {
            \__modiagram_AO_place_electron:nx {1}
              { \tl_use:N \l__modiagram_AO_x_down_electron_tl }
          }
        \__modiagram_AO:ff
          { \tl_use:N \l__modiagram_AO_px_electron_tl }
          { \tl_use:N \l__modiagram_AO_x_electron_color_tl }
      } ;
    \__modiagram_AO_get_orbital_x_label:o
      {
        \tl_use:N \l__modiagram_AO_name_tl
        \bool_if:NT \l__modiagram_AO_p_bool { x }
      }
    \bool_if:NT \l__modiagram_AO_p_bool
      {
        \__modiagram_node:n { inner~sep=0 , outer~sep=0 }
          ( \tl_use:N \l__modiagram_AO_name_tl y )
          at
          (
            \l__modiagram_AO_position_dim + \l__modiagram_atom_py_shift_dim ,
            \l__modiagram_AO_dim
          )
          {
            \bool_if:nT
              {
                \l__modiagram_AO_up_electron_bool ||
                \l__modiagram_AO_y_up_electron_bool
              }
              {
                \__modiagram_AO_place_electron:nx {0}
                  { \tl_use:N \l__modiagram_AO_y_up_electron_tl }
              }
            \bool_if:nT
              {
                \l__modiagram_AO_down_electron_bool ||
                \l__modiagram_AO_y_down_electron_bool
              }
              {
                \__modiagram_AO_place_electron:nx {1}
                  { \tl_use:N \l__modiagram_AO_y_down_electron_tl }
              }
            \__modiagram_AO:ff
              { \tl_use:N \l__modiagram_AO_py_electron_tl }
              { \tl_use:N \l__modiagram_AO_y_electron_color_tl }
          } ;
        % `o' expansion doesn't seem to make sense here:
        \__modiagram_AO_get_orbital_y_label:o
          { \tl_use:N \l__modiagram_AO_name_tl y }
        \__modiagram_node:n { inner~sep=0 , outer~sep=0 }
          ( \tl_use:N \l__modiagram_AO_name_tl z )
          at
          (
            \l__modiagram_AO_position_dim + \l__modiagram_atom_pz_shift_dim ,
            \l__modiagram_AO_dim
          )
          {
            \bool_if:nT
              {
                \l__modiagram_AO_up_electron_bool ||
                \l__modiagram_AO_z_up_electron_bool
              }
              {
                \__modiagram_AO_place_electron:nx {0}
                  { \tl_use:N \l__modiagram_AO_z_up_electron_tl }
              }
            \bool_if:nT
              {
                \l__modiagram_AO_down_electron_bool ||
                \l__modiagram_AO_z_down_electron_bool
              }
              {
                \__modiagram_AO_place_electron:nx {1}
                  { \tl_use:N \l__modiagram_AO_z_down_electron_tl }
              }
            \__modiagram_AO:ff
              { \tl_use:N \l__modiagram_AO_pz_electron_tl }
              { \tl_use:N \l__modiagram_AO_z_electron_color_tl }
          } ;
        \__modiagram_AO_get_orbital_z_label:o
          { \tl_use:N \l__modiagram_AO_name_tl z }
      }
    \group_end:
  }

% --------------------------------------------------------------------------- %
% connect arbitrary AOs / MOs
\NewDocumentCommand \connect {m}
  {
    \clist_map_variable:nNn {#1} \l__modiagram_tmpa_tl
      {
        \tl_remove_all:Nn \l__modiagram_tmpa_tl {~}
        \tl_if_blank:VF \l__modiagram_tmpa_tl
          {
            \__modiagram_detect_nodes:x { \l__modiagram_tmpa_tl }
             \tl_if_in:NnF \l__modiagram_node_A_tl {.}
               { \tl_put_right:Nn \l__modiagram_node_A_tl {.east} }
             \tl_if_in:NnF \l__modiagram_node_B_tl {.}
               { \tl_put_right:Nn \l__modiagram_node_B_tl {.west} }
            \__modiagram_draw_connection:xxx
              { \tl_use:N \l__modiagram_lines_tl }
              { \tl_use:N \l__modiagram_node_A_tl }
              { \tl_use:N \l__modiagram_node_B_tl }
          }
      }
  }

\NewDocumentCommand \__modiagram_detect_nodes:n { > { \SplitArgument {1} {&} } m }
  { \__modiagram_detect_nodes_aux_i:nn #1 }
\cs_generate_variant:Nn \__modiagram_detect_nodes:n {x}

\cs_new_protected:Npn \__modiagram_detect_nodes_aux_i:nn #1#2
  {
    \tl_set:Nn \l__modiagram_node_A_tl {#1}
    \tl_set:Nn \l__modiagram_node_B_tl {#2}
  }

\cs_new:Npn \__modiagram_draw_connection:nnn #1#2#3
  { \draw[#1] (#2) -- (#3) ; }
\cs_generate_variant:Nn \__modiagram_draw_connection:nnn {xxx}

% --------------------------------------------------------------------------- %
% orbital labels
\prop_new:N \l__modiagram_orbital_label_prop
\tl_set:Nn \l__modiagram_orbital_label_fontsize_tl { \small }

\cs_new:Npn \__modiagram_orbital_label_set:n #1
  { \keys_set:nn {modiagram/label} {#1} }

\cs_new_protected:Npn \__modiagram_get_orbital_label:n #1
  {
    \tl_clear:N \l__modiagram_tmpa_tl
    \tl_clear:N \l__modiagram_tmpb_tl
    \prop_get:NnNT \l__modiagram_orbital_label_prop {#1} \l__modiagram_tmpa_tl
      {
        \__modiagram_node:x
          { below , \tl_use:N \l__modiagram_orbital_label_style_tl }
          at (#1.south)
          {
            \tl_use:N \l__modiagram_orbital_label_fontsize_tl
            \tl_use:N \l__modiagram_tmpa_tl
          } ;
      }
  }

\keys_define:nn {modiagram/label}
  {
    1sleft   .code:n =
      { \prop_put:Nnn \l__modiagram_orbital_label_prop {1sleft} {#1} } ,
    1sright  .code:n =
      { \prop_put:Nnn \l__modiagram_orbital_label_prop {1sright} {#1} } ,
    2sleft   .code:n =
      { \prop_put:Nnn \l__modiagram_orbital_label_prop {2sleft} {#1} } ,
    2sright  .code:n =
      { \prop_put:Nnn \l__modiagram_orbital_label_prop {2sright} {#1} } ,
    2pxleft  .code:n =
      { \prop_put:Nnn \l__modiagram_orbital_label_prop {2pxleft} {#1} } ,
    2pxright .code:n =
      { \prop_put:Nnn \l__modiagram_orbital_label_prop {2pxright} {#1} } ,
    2pyleft  .code:n =
      { \prop_put:Nnn \l__modiagram_orbital_label_prop {2pyleft} {#1} } ,
    2pyright .code:n =
      { \prop_put:Nnn \l__modiagram_orbital_label_prop {2pyright} {#1} } ,
    2pzleft  .code:n =
      { \prop_put:Nnn \l__modiagram_orbital_label_prop {2pzleft} {#1} } ,
    2pzright .code:n =
      { \prop_put:Nnn \l__modiagram_orbital_label_prop {2pzright} {#1} } ,
    1sigma   .code:n =
      { \prop_put:Nnn \l__modiagram_orbital_label_prop {1sigma} {#1} } ,
    1sigma*  .code:n =
      { \prop_put:Nnn \l__modiagram_orbital_label_prop {1sigma*} {#1} } ,
    2sigma   .code:n =
      { \prop_put:Nnn \l__modiagram_orbital_label_prop {2sigma} {#1} } ,
    2sigma*  .code:n =
      { \prop_put:Nnn \l__modiagram_orbital_label_prop {2sigma*} {#1} } ,
    2psigma  .code:n =
      { \prop_put:Nnn \l__modiagram_orbital_label_prop {2psigma} {#1} } ,
    2psigma* .code:n =
      { \prop_put:Nnn \l__modiagram_orbital_label_prop {2psigma*} {#1} } ,
    2piy     .code:n =
      { \prop_put:Nnn \l__modiagram_orbital_label_prop {2piy} {#1} } ,
    2piy*    .code:n =
      { \prop_put:Nnn \l__modiagram_orbital_label_prop {2piy*} {#1} } ,
    2piz     .code:n =
      { \prop_put:Nnn \l__modiagram_orbital_label_prop {2piz} {#1} } ,
    2piz*    .code:n =
      { \prop_put:Nnn \l__modiagram_orbital_label_prop {2piz*} {#1} }
  }

\char_set_catcode_math_subscript:N \@
\cs_new_protected:Npn \__modiagram_set_labels:
  {
    \bool_if:NT \l__modiagram_AO_labels_bool
      {
        \prop_put:Nnn \l__modiagram_orbital_label_prop {1sleft} { 1s }
        \prop_put:Nnn \l__modiagram_orbital_label_prop {1sright} { 1s }
        \prop_put:Nnn \l__modiagram_orbital_label_prop {2sleft} { 2s }
        \prop_put:Nnn \l__modiagram_orbital_label_prop {2sright} { 2s }
        \prop_put:Nnn \l__modiagram_orbital_label_prop {2pxleft} { 2p$@x$ }
        \prop_put:Nnn \l__modiagram_orbital_label_prop {2pxright} { 2p$@x$ }
        \prop_put:Nnn \l__modiagram_orbital_label_prop {2pyleft} { 2p$@y$ }
        \prop_put:Nnn \l__modiagram_orbital_label_prop {2pyright} { 2p$@y$ }
        \prop_put:Nnn \l__modiagram_orbital_label_prop {2pzleft} { 2p$@z$ }
        \prop_put:Nnn \l__modiagram_orbital_label_prop {2pzright} { 2p$@z$ }
        \prop_put:Nnn \l__modiagram_orbital_label_prop {1sigma} { 1\chemsigma$@s$ }
        \prop_put:Nnn \l__modiagram_orbital_label_prop {1sigma*} { 1\chemsigma$^*@s$ }
        \prop_put:Nnn \l__modiagram_orbital_label_prop {2sigma} { 2\chemsigma$@s$ }
        \prop_put:Nnn \l__modiagram_orbital_label_prop {2sigma*} { 2\chemsigma$^*@s$ }
        \prop_put:Nnn \l__modiagram_orbital_label_prop {2psigma} { 2\chemsigma$@x$ }
        \prop_put:Nnn \l__modiagram_orbital_label_prop {2psigma*} { 2\chemsigma$^*@x$ }
        \prop_put:Nnn \l__modiagram_orbital_label_prop {2piy} { 2\chempi$@y$ }
        \prop_put:Nnn \l__modiagram_orbital_label_prop {2piy*} { 2\chempi$^*@y$ }
        \prop_put:Nnn \l__modiagram_orbital_label_prop {2piz} { 2\chempi$@z$ }
        \prop_put:Nnn \l__modiagram_orbital_label_prop {2piz*} { 2\chempi$^*@z$ }
      }
  }
\char_set_catcode_letter:N \@

% --------------------------------------------------------------------------- %
% electron colours
\prop_new:N \l__modiagram_electron_color_prop
\tl_new:N \l__modiagram_electron_current_color_tl

\cs_new:Npn \__modiagram_electron_color_set:n #1
  { \keys_set:nn {modiagram/color} {#1} }

\cs_new_protected:Npn \__modiagram_get_electron_color:n #1
  {
    \tl_clear:N \l__modiagram_tmpa_tl
    \tl_clear:N \l__modiagram_tmpb_tl
    \prop_get:NnNT \l__modiagram_electron_color_prop {#1} \l__modiagram_tmpa_tl
      { \tl_set_eq:NN \l__modiagram_electron_current_color_tl \l__modiagram_tmpa_tl }
  }

\keys_define:nn {modiagram/color}
  {
    1sleft   .code:n =
      { \prop_put:Nnn \l__modiagram_electron_color_prop {1sleft} {#1} } ,
    1sright  .code:n =
      { \prop_put:Nnn \l__modiagram_electron_color_prop {1sright} {#1} } ,
    2sleft   .code:n =
      { \prop_put:Nnn \l__modiagram_electron_color_prop {2sleft} {#1} } ,
    2sright  .code:n =
      { \prop_put:Nnn \l__modiagram_electron_color_prop {2sright} {#1} } ,
    2pxleft  .code:n =
      { \prop_put:Nnn \l__modiagram_electron_color_prop {2pxleft} {#1} } ,
    2pxright .code:n =
      { \prop_put:Nnn \l__modiagram_electron_color_prop {2pxright} {#1} } ,
    2pyleft  .code:n =
      { \prop_put:Nnn \l__modiagram_electron_color_prop {2pyleft} {#1} } ,
    2pyright .code:n =
      { \prop_put:Nnn \l__modiagram_electron_color_prop {2pyright} {#1} } ,
    2pzleft  .code:n =
      { \prop_put:Nnn \l__modiagram_electron_color_prop {2pzleft} {#1} } ,
    2pzright .code:n =
      { \prop_put:Nnn \l__modiagram_electron_color_prop {2pzright} {#1} } ,
    1sigma   .code:n =
      { \prop_put:Nnn \l__modiagram_electron_color_prop {1sigma} {#1} } ,
    1sigma*  .code:n =
      { \prop_put:Nnn \l__modiagram_electron_color_prop {1sigma*} {#1} } ,
    2sigma   .code:n =
      { \prop_put:Nnn \l__modiagram_electron_color_prop {2sigma} {#1} } ,
    2sigma*  .code:n =
      { \prop_put:Nnn \l__modiagram_electron_color_prop {2sigma*} {#1} } ,
    2psigma  .code:n =
      { \prop_put:Nnn \l__modiagram_electron_color_prop {2psigma} {#1} } ,
    2psigma* .code:n =
      { \prop_put:Nnn \l__modiagram_electron_color_prop {2psigma*} {#1} } ,
    2piy     .code:n =
      { \prop_put:Nnn \l__modiagram_electron_color_prop {2piy} {#1} } ,
    2piy*    .code:n =
      { \prop_put:Nnn \l__modiagram_electron_color_prop {2piy*} {#1} } ,
    2piz     .code:n =
      { \prop_put:Nnn \l__modiagram_electron_color_prop {2piz} {#1} } ,
    2piz*    .code:n =
      { \prop_put:Nnn \l__modiagram_electron_color_prop {2piz*} {#1} }
  }

% --------------------------------------------------------------------------- %
% electron positions
\prop_new:N \l__modiagram_electron_up_prop

\cs_new:Npn \__modiagram_electron_up_set:n #1
  { \keys_set:nn {modiagram/up-pos} {#1} }

\cs_new_protected:Npn \__modiagram_get_electron_up:n #1
  {
    \tl_clear:N \l__modiagram_tmpa_tl
    \tl_clear:N \l__modiagram_tmpb_tl
    \prop_get:NnNT \l__modiagram_electron_up_prop {#1} \l__modiagram_tmpa_tl
      { \tl_set_eq:NN \l__modiagram_AO_up_electron_tl \l__modiagram_tmpa_tl }
  }

\keys_define:nn {modiagram/up-pos}
  {
    1sleft   .code:n =
      { \prop_put:Nnn \l__modiagram_electron_up_prop {1sleft} {#1} } ,
    1sright  .code:n =
      { \prop_put:Nnn \l__modiagram_electron_up_prop {1sright} {#1} } ,
    2sleft   .code:n =
      { \prop_put:Nnn \l__modiagram_electron_up_prop {2sleft} {#1} } ,
    2sright  .code:n =
      { \prop_put:Nnn \l__modiagram_electron_up_prop {2sright} {#1} } ,
    2pxleft  .code:n =
      { \prop_put:Nnn \l__modiagram_electron_up_prop {2pxleft} {#1} } ,
    2pxright .code:n =
      { \prop_put:Nnn \l__modiagram_electron_up_prop {2pxright} {#1} } ,
    2pyleft  .code:n =
      { \prop_put:Nnn \l__modiagram_electron_up_prop {2pyleft} {#1} } ,
    2pyright .code:n =
      { \prop_put:Nnn \l__modiagram_electron_up_prop {2pyright} {#1} } ,
    2pzleft  .code:n =
      { \prop_put:Nnn \l__modiagram_electron_up_prop {2pzleft} {#1} } ,
    2pzright .code:n =
      { \prop_put:Nnn \l__modiagram_electron_up_prop {2pzright} {#1} } ,
    1sigma   .code:n =
      { \prop_put:Nnn \l__modiagram_electron_up_prop {1sigma} {#1} } ,
    1sigma*  .code:n =
      { \prop_put:Nnn \l__modiagram_electron_up_prop {1sigma*} {#1} } ,
    2sigma   .code:n =
      { \prop_put:Nnn \l__modiagram_electron_up_prop {2sigma} {#1} } ,
    2sigma*  .code:n =
      { \prop_put:Nnn \l__modiagram_electron_up_prop {2sigma*} {#1} } ,
    2psigma  .code:n =
      { \prop_put:Nnn \l__modiagram_electron_up_prop {2psigma} {#1} } ,
    2psigma* .code:n =
      { \prop_put:Nnn \l__modiagram_electron_up_prop {2psigma*} {#1} } ,
    2piy     .code:n =
      { \prop_put:Nnn \l__modiagram_electron_up_prop {2piy} {#1} } ,
    2piy*    .code:n =
      { \prop_put:Nnn \l__modiagram_electron_up_prop {2piy*} {#1} } ,
    2piz     .code:n =
      { \prop_put:Nnn \l__modiagram_electron_up_prop {2piz} {#1} } ,
    2piz*    .code:n =
      { \prop_put:Nnn \l__modiagram_electron_up_prop {2piz*} {#1} }
  }

\prop_new:N \l__modiagram_electron_down_prop

\cs_new:Npn \__modiagram_electron_down_set:n #1
  { \keys_set:nn {modiagram/down-pos} {#1} }

\cs_new_protected:Npn \__modiagram_get_electron_down:n #1
  {
    \tl_clear:N \l__modiagram_tmpa_tl
    \tl_clear:N \l__modiagram_tmpb_tl
    \prop_get:NnNT \l__modiagram_electron_down_prop {#1} \l__modiagram_tmpa_tl
      { \tl_set_eq:NN \l__modiagram_AO_down_electron_tl \l__modiagram_tmpa_tl }
  }

\keys_define:nn {modiagram/down-pos}
  {
    1sleft   .code:n =
      { \prop_put:Nnn \l__modiagram_electron_down_prop {1sleft} {#1} } ,
    1sright  .code:n =
      { \prop_put:Nnn \l__modiagram_electron_down_prop {1sright} {#1} } ,
    2sleft   .code:n =
      { \prop_put:Nnn \l__modiagram_electron_down_prop {2sleft} {#1} } ,
    2sright  .code:n =
      { \prop_put:Nnn \l__modiagram_electron_down_prop {2sright} {#1} } ,
    2pxleft  .code:n =
      { \prop_put:Nnn \l__modiagram_electron_down_prop {2pxleft} {#1} } ,
    2pxright .code:n =
      { \prop_put:Nnn \l__modiagram_electron_down_prop {2pxright} {#1} } ,
    2pyleft  .code:n =
      { \prop_put:Nnn \l__modiagram_electron_down_prop {2pyleft} {#1} } ,
    2pyright .code:n =
      { \prop_put:Nnn \l__modiagram_electron_down_prop {2pyright} {#1} } ,
    2pzleft  .code:n =
      { \prop_put:Nnn \l__modiagram_electron_down_prop {2pzleft} {#1} } ,
    2pzright .code:n =
      { \prop_put:Nnn \l__modiagram_electron_down_prop {2pzright} {#1} } ,
    1sigma   .code:n =
      { \prop_put:Nnn \l__modiagram_electron_down_prop {1sigma} {#1} } ,
    1sigma*  .code:n =
      { \prop_put:Nnn \l__modiagram_electron_down_prop {1sigma*} {#1} } ,
    2sigma   .code:n =
      { \prop_put:Nnn \l__modiagram_electron_down_prop {2sigma} {#1} } ,
    2sigma*  .code:n =
      { \prop_put:Nnn \l__modiagram_electron_down_prop {2sigma*} {#1} } ,
    2psigma  .code:n =
      { \prop_put:Nnn \l__modiagram_electron_down_prop {2psigma} {#1} } ,
    2psigma* .code:n =
      { \prop_put:Nnn \l__modiagram_electron_down_prop {2psigma*} {#1} } ,
    2piy     .code:n =
      { \prop_put:Nnn \l__modiagram_electron_down_prop {2piy} {#1} } ,
    2piy*    .code:n =
      { \prop_put:Nnn \l__modiagram_electron_down_prop {2piy*} {#1} } ,
    2piz     .code:n =
      { \prop_put:Nnn \l__modiagram_electron_down_prop {2piz} {#1} } ,
    2piz*    .code:n =
      { \prop_put:Nnn \l__modiagram_electron_down_prop {2piz*} {#1} }
  }

% --------------------------------------------------------------------------- %
% energy axis
\dim_new:N \l_energy_axis_dim
\dim_set:Nn \l_energy_axis_dim {0pt}

\tl_new:N \l__modiagram_Eaxis_head_tl
\tl_set:Nn \l__modiagram_Eaxis_head_tl {latex}
\tl_new:N \l__modiagram_Eaxis_title_tl

\bool_new:N \l__modiagram_Eaxis_title_bool
\bool_set_false:N \l__modiagram_Eaxis_title_bool

\NewDocumentCommand \__modiagram_EnergyAxis: {o}
  {
    \IfNoValueF {#1} { \keys_set:nn {modiagram/axis} {#1} }
    \__modiagram_draw:f { - \tl_use:N \l__modiagram_Eaxis_head_tl }
      (\l_energy_axis_dim , \g__modiagram_min_energy_dim - 1cm )
      --
      (\l_energy_axis_dim , \g__modiagram_max_energy_dim + 1cm ) 
      \bool_if:NT \l__modiagram_Eaxis_title_bool
        { node [ above ] { \tl_use:N \l__modiagram_Eaxis_title_tl } }
    ;
  }

\keys_define:nn {modiagram/axis}
  {
    title .code:n =
      {
        \bool_set_true:N \l__modiagram_Eaxis_title_bool
        \tl_set:Nn \l__modiagram_Eaxis_title_tl {#1}
      } ,
    title .default:n = energy ,
    head  .tl_set:N  = \l__modiagram_Eaxis_head_tl ,
    head  .default:n = >
  }

% --------------------------------------------------------------------------- %
% tikz helper functions
\cs_new:Npn \__modiagram_node:n #1
  { \node [#1] }
\cs_generate_variant:Nn \__modiagram_node:n {f,x}

\cs_new:Npn \__modiagram_draw:n #1
  { \draw [#1] }
\cs_generate_variant:Nn \__modiagram_draw:n {f}

\cs_new:Npn \__modiagram_tikz:nn #1
  { \tikz [#1] }
\cs_generate_variant:Nn \__modiagram_tikz:nn {f,x}

\cs_new:Npn \__modiagram_tikzpicture:n #1
  { \tikzpicture [#1] }
\cs_generate_variant:Nn \__modiagram_tikzpicture:n {f}

\file_input_stop:

% --------------------------------------------------------------------------- %
Version history
2011/09/25 version 0.1  - first upload to CTAN
2011/10/27 version 0.2  - \atom, \molecule, \AO, \EnergyAxis only defined within
                          the modiagram environment
                        - bugfix in energy splitting process
                        - bugfix: wrong labelling of plain empty orbitals
                        - different values for energy gain and loss possible
                        - default values for the AO and MO; if empty values are
                          used, the ";" and "," can be omitted
                        - default node names for the arbitrary AOs
                        - horizontal shift of electrons possible
                        - customization of atom- and molecule names possible
2013/04/28 version 0.2c - fix: adapt to deprecated expl3 command
2014/05/12 version 0.2d - fix: adapt to deprecated expl3 command
                        - change dependency from `textgreek' to `chemgreek'
2014/06/28 version 0.2e - fix: adapt to deprecated expl3 command
2015/07/19 version 0.2f - fix: adapt to deprecated expl3 command
2015/09/23 version 0.2g - fix: adapt to deprecated expl3 command
2019/10/31 version 0.3  - correct wrong expl3 code
                        - rename `MOdiagram' into `modiagram'
                        - rename \MOsetup into \setmodiagram
2020/11/08 version 0.3a - fix spurious \show

TODO
- reduce code duplication
- reduce usage of f expansion
- get rid of hard-coded defaults
- 3s, 3p , 3sMO , 3pMO , 4s , sp3
- energy axis: ticks? at orbital heights?
- electron configuration
- localisation for the energy axis title using the translator package
- Default-Einheit (bislang cm) ändern können?