Exchange Effects in the Degenerate Perturbation Theory of Intermolecular Potentials

Abstract

The contribution of electron exchange to intermolecular potentials is studied using the operator form of perturbation theory, when the noninteracting system of atoms or molecules is degenerate. A symmetry‐adapted, degenerate perturbation theory is developed, including the almost‐degeneracy case. Perturbation expansions are given up to second order in the energy. Differential equations for the perturbed wave‐functions are obtained, as well as their associated variational principles.