code: def _set_residue_map(self):
          
          self.map_residue_to_mol = {}
          lookup = {}
          for idx, mol in enumerate(self.mols):
              if not mol.formula in lookup:
                  mol.translate_sites(indices=range(len(mol)),
                                      vector=-mol.center_of_mass)
                  lookup[mol.formula] = mol.copy()
              self.map_residue_to_mol["ml{}".format(idx + 1)] = lookup[mol.formula]
docs: map each residue to the corresponding molecule.