EXCHANGE ENERGY OF H2+ CALCULATED FROM POLARIZATION PERTURBATION-THEORY AND THE HOLSTEIN-HERRING METHOD

Tang, Toennies, and Yiu have shown that unsymmetrized Rayleigh-Schrodinger perturbation theory can be used to calculate the leading terms in the ground-state exchange energy of H-2+ using the method of Holstein and Herring. We clarify this seemingly puzzling result by showing that their approximation to the polarization wave function is exactly the leading term of the Herring-Holstein function. We also show that a generalization of their method yields the leading asymptotic exchange energy terms for excited states, thereby suggesting the existence of a general underlying property and the applicability of the method to many-electron systems.