code: def _add_node(self, idx, unique_idx, frac_coords):
          
          self.nodes[idx] = {'unique_idx': unique_idx, 'frac_coords': frac_coords}
docs: Add information about a node describing a critical point.
  
          :param idx: unique index
          :param unique_idx: index of unique CriticalPoint,
          used to look up more information of point (field etc.)
          :param frac_coord: fractional co-ordinates of point
          :return: