docs: def neighbor_atoms(self, atom_symbol=None):
          """Access neighbor atoms.
  
          :param str atom_symbol: Atom symbol.
          :return: List of neighbor atoms.
          :rtype: :py:class:`list`.
          """
          if not atom_symbol:
              return self.neighbors
          else:
              return [atom for atom in self.neighbors if atom['atom_symbol'] == atom_symbol]