Ab initio studies of the intermolecular interactions between two hydrogen molecules near the van der Waals minimum from a perturbation procedure using biorthogonal orbitals

Abstract

The intermolecular energy between two hydrogen molecules near the van der Waals minimum has been computed from a perturbative procedure using biorthogonal orbitals. The zeroth-order hamiltonian possesses the proper symmetry with respect to the intermolecular electron permutation. This procedure allows us to study separately the effect of the overlap and the effect of the exchange of the electrons on the different terms of the perturbation series. It is seen that the overlap cannot be neglected in the first-order term at the van der Waals minimum, and can affect the dispersion energy by as much as 20 per cent. This method can be generalized for larger systems and reliable approximations are considered in order to shorten the computation time.