docs: def get_atom_type_symbol(cls,calc,**kwargs):
          """
          Returns a list of atom types. Each atom site MUST occur only
          once in this list. List MUST be sorted.
          """
          parameters = calc.out.output
          dictionary = parameters.get_dict()
          if 'basis_set' not in dictionary.keys():
              return None
          return sorted(dictionary['basis_set'].keys())