Possible artifacts occurring in the calculation of intermolecular energies from delocalized pictures

The intermolecular energy between two identical subsystems may be calculated from symmetrydelocalized MO's resulting for instance from a preliminary SCF calculation of the supersystem. Then each second-order energy correction mixes intramolecular correlation,R^–6 intermolecular dispersion energy, andR^–3 components. TheR^–3 components disappear through subtle cancellations. The shifted Epstein-Nesbet energy denominators introduce an artificial second-order intermolecularR^–1 component, which would be cancelled by off-diagonal third-order terms, as well as a bad asymptotic limit at infinite distances. TheR^–1 artifact will also occur in strong symmetrical chemical bonds calculated in the Epstein-Nesbet perturbation scheme from delocalized MO's. These defects will occur in all variational approximate CI techniques which neglect off-diagonal elements between delocalized doubly excited determinants. These artifacts are avoided when using the Moller-Plesset definition of the zeroth order Hamiltonian or when starting from (SCF)localized MO's (even in the Epstein-Nesbet perturbation). The discussion is exemplified on an accurateab initio calculation of the Ar₂ molecule.