Send the species multiplicity to Arkane's input file (#804)

To allow Arkane to process (and differentiate between) singlet and
triplet forms of the same structure.
See ReactionMechanismGenerator/RMG-Py#2854

Author: alongd

arc/statmech/arkane.py

@@ -50,7 +50,7 @@
 % for spc in species_list:
 % if spc['smiles']:
 species('${spc['label']}', '${spc['path']}'${spc['pdep_data'] if 'pdep_data' in spc else ''},
-        structure=SMILES('${spc['smiles']}'))
+        structure=SMILES('${spc['smiles']}'), spinMultiplicity=${spc['multiplicity']})
 % else:
 species('${spc['label']}', '${spc['path']}'${spc['pdep_data'] if 'pdep_data' in spc else ''})
 % endif
@@ -325,7 +325,9 @@ def render_arkane_input_template(self,
             if e0_only or spc.compute_thermo:
                 species_list.append({'label': spc.label,
                                      'path': spc.yml_path or os.path.join(statmech_dir, 'species', f'{spc.label}.py'),
-                                     'smiles': spc.mol.copy(deep=True).to_smiles() if not spc.is_ts else ''})
+                                     'smiles': spc.mol.copy(deep=True).to_smiles() if not spc.is_ts else '',
+                                     'multiplicity': spc.multiplicity,
+                                     })
         ts_list = [{'label': rxn.ts_species.label,
                      'path': rxn.ts_species.yml_path or os.path.join(statmech_dir, 'TSs', f'{rxn.ts_species.label}.py')}
                      for rxn in self.reactions] if self.reactions else list()

arc/statmech/arkane_test.py

@@ -154,8 +154,8 @@ def test_generate_arkane_input(self):
         input_path = os.path.join(statmech_dir, 'input.py')
         expected_lines = ["#!/usr/bin/env python",
                           "title = 'Arkane kinetics calculation'",
-                          "        structure=SMILES('C[NH]'))",
-                          "        structure=SMILES('[CH2]N'))",
+                          "        structure=SMILES('C[NH]'), spinMultiplicity=2)",
+                          "        structure=SMILES('[CH2]N'), spinMultiplicity=2)",
                           "    label='CH3NH <=> CH2NH2',",
                           "    reactants=['CH3NH'],",
                           "    products=['CH2NH2'],",