POSSIBLE ARTIFACTS OCCURRING IN THE CALCULATION OF INTER-MOLECULAR ENERGIES FROM DELOCALIZED PICTURES

The intermolecular energy between two identical subsystems may be calculated from symmetry delocalized 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, and R-3 components. The R-3 components disappear through subtle cancellations. The shifted Epstein-Nesbet energy denominators introduce an artificial second-order intermolecular R-1 component, which would be cancelled by off-diagonal third-order terms, as well as a bad asymptotic limit at infinite distances. The R-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 accurate ab initio calculation of the Ar2 molecule. © 1979 Springer-Verlag.