Ab initio intermolecular energies between Li+ and H2 near the van der waals minimum: comparison between a perturbative procedure and an SCF supermolecule treatment

The electrostatic, exchange, polarization, and dispersion energies are computed without using the multipole expansion. It is seen that the overlap between the orbitals of the interacting molecules must be taken into account to describe qualitatively the first order term. The neglect of overlap in the polarization term overestimates the attractive energy around the van der Waals minimum. An interpretation of the polarization phenomena in terms of molecular orbitals is proposed. The results are compared with SCF calculations and the use of small basis sets is considered.