Looking at Chemical Bonding from Coulomb and Exchange Correlations in NAOs

Abstract

Coulomb and exchange correlations, providing information for the interactions of antiparallel and parallel spin electrons, respectively, are investigated in orbitals appropriate for population analysis, such as the natural atomic orbitals (NAOs). In the proposed analysis, both correlations are treated on an equal footing in configuration interaction (CI) (or Hartree−Fock) levels, but an emphasis is given for coulomb correlations and their physical meaning. It is stressed that the two-center interactions of antiparallel spin electrons can be “repulsive” and “attractive” (as a direct consequence of chemical bonding), but the former are less important than the latter; their relative importance is determined by the magnitude of one-center interactions. These globally attractive two-center interactions are balanced by the repulsive one-center interactions. These conclusions are general and hold for any molecular system, under the only assumption that the one-center interactions are repulsive. Molecular orbital...