| Abstract
| - Coulomb and exchange correlations, providing information for the interactions of antiparallel and parallelspin electrons, respectively, are investigated in orbitals appropriate for population analysis, such as the naturalatomic orbitals (NAOs). In the proposed analysis, both correlations are treated on an equal footing inconfiguration interaction (CI) (or Hartree−Fock) levels, but an emphasis is given for coulomb correlationsand 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 importantthan the latter; their relative importance is determined by the magnitude of one-center interactions. Theseglobally attractive two-center interactions are balanced by the repulsive one-center interactions. Theseconclusions are general and hold for any molecular system, under the only assumption that the one-centerinteractions are repulsive. Molecular orbital wave functions for the cis-butadiene molecule (in variousapproximations levels) are used to illustrate the relative role of coulomb and exchange interactions in chemicalbonding. The magnitudes of exchange interactions are significantly larger (2−4 times) than those of thecoulomb interactions; the signs of two-center coulomb and exchange correlations are opposite. Even thoughthe CI is very crucial for coulomb interactions, the exchange interactions, in general, are not so sensitive. Theprovided description for chemical bonding is consistent with usual chemical pictures involving electron pairs.
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