Ab initio calculation of the first order term of the intermolecular energy near the van der Waals minimum

The first order term of the intermolecular energies between two hydrogen molecules and between Li⁺ and H₂ has been computed by three different methods: two of them are based on a perturbative procedure, including or neglecting the overlap between the orbitals of the interacting molecules or atoms in the calculation of the electrostatic and exchange terms. We can then study the effect of the overlap on each of these terms. The third method is the SCF supermolecule treatment which provides results in very good agreement with the perturbative procedure including the overlap. TheT configuration in the case of two hydrogen molecules and theC _2v configuration for Li⁺+H₂ are stable with respect to the first order term.