Label-free exchange perturbation approximation for the collision-induced dipole of He⋅⋅⋅H

Abstract

A label-free exchange perturbation method is used in this work to calculate the net dipole μ(R) of interacting He and H atoms as a function of internuclear separation R. In the label-free formalism, the unperturbed Hamiltonian and perturbation terms are constructed so that each is invariant with respect to exchange of electrons between the interacting atoms; then a direct Rayleigh–Schrödinger perturbation expansion with a fully antisymmetrized set of zeroth-order wave functions yields the interaction energy and collision-induced properties. Good agreement with accurate ab initio results for the He ⋅⋅⋅ H dipole is obtained when a long-range dispersion contribution (varying as R−7) is added to the first-order overlap and exchange contributions, computed with an extended Gaussian basis. Near the van der Waals minimum and at short range the dipole vector points from the H nucleus to the He nucleus, but at long range the dipole orientation is reversed. Between R=4.0 and 8.0 a.u., the dipole decreases rapidly (with a roughly exponential R dependence) as the internuclear distance increases.