THE PERTURBATION CALCULATION OF VAN-DER-WAALS POTENTIALS

The unsymmetrized perturbation theory for interaction potentials is reformulated in such a way that the overlap and exchange effects can be taken into account in a satisfactory and conceptually simple way. This formulation, known as the generalized Heitler-London theory, its shown to be valid regardless of the ultimate limit to which the polarization approximation converges. Within the framework of this theory, the van der Waals potential of the triplet H-2(3SIGMA(u)) state is calculated and shown to be in excellent agreement with the exact ab initio results. Both the exchange energy and the polarization energy are obtained from a perturbation calculation.