code: def _describe_atom(topology, index):
      
      at = topology.atom(index)
      if topology.n_chains > 1:
          return "%s %i %s %i %i" % (at.residue.name, at.residue.resSeq, at.name, at.index, at.residue.chain.index )
      else:
          return "%s %i %s %i"    % (at.residue.name, at.residue.resSeq, at.name, at.index)
docs: Returns a string describing the given atom
  
      :param topology:
      :param index:
      :return: