docs: def parameter_struct_dict(self):
          """Dictionary containing PyAtomData structs for the force field."""
          if self._parameter_struct_dict is None:
              self._parameter_struct_dict = self._make_ff_params_dict()
          elif self.auto_update_f_params:
              new_hash = hash(
                  tuple([tuple(item)
                         for sublist in self.values()
                         for item in sublist.values()]))
              if self._old_hash != new_hash:
                  self._parameter_struct_dict = self._make_ff_params_dict()
                  self._old_hash = new_hash
          return self._parameter_struct_dict