A variational principle for the Heitler-London intermolecular interaction energy

Abstract

An extension of the Delves variational principle is used to find an expression for the Heitler-London (exchange) interaction energy which differs from the exact results by second order terms. The auxiliary functions so introduced are found by relating the variational principle to Symmetry-Adapted Double Perturbation theory and, with neglect of a term likely to be small, this expression is the same as that usually used. From these considerations two criteria for the optimization of wavefunctions can be found. The criteria are applied to the Helium-Helium and Neon-Neon interactions using minimal basis wavefunctions and, by allowing the parameter values to vary with internuclear distance, significant improvements in the interaction energies are obtained, in particular those for Helium-Helium agreeing with accurate SCF results.