Generalization of the "Linear Theory" of Configuration Interaction

The "linear" theory of configuration interaction has met with considerable success in giving a phenomenological description of the energy levels of equivalent-electron systems. In the present paper the generalization of the "linear" theory to configurations containing nonequivalent electrons is examined from the point of view of second-order perturbation theory. It is shown that most second-order electrostatic interactions can be phenomenologically described by the first-order terms of a set of effective two-body scalar interactions. The significance of these interactions in atomic energy-level calculations is considered.