CHARGE FLUCTUATIONS AND CORRELATION STRENGTH IN CHEMICAL-BONDS - FIRST-ROW HOMONUCLEAR DIATOMIC-MOLECULES

We investigate, by means of ab initio calculations, the strength of el ectron correlations within covalent bonds: the first-row homonuclear d iatomics serve as test cases. As an appropriate measure of the correla tion strength, we introduce the reduction of the mean-square deviation s of the electronic charges in localized orbitals forming a bond. A re cently developed population analysis in terms of local operators deriv ed from localized molecular orbitals is thereby used. The correlation- strength parameter depends only weakly on dynamical correlations as te st calculations demonstrate. Therefore, the full-valence complete acti ve space self-consistent field (CASSCF) approximation is applied in or der to study the changes in the correlation strength with changing bon d length for different types of bonds. A number of simple rules emerge from this discussion. In addition, we show that charge fluctuations a re not only a reliable measure of intrabond correlation effects, but a lso can be used to monitor intraatomic quasi-degeneracy effects as wel l as the interdependence within multiple bonds. (C) 1998 John Wiley & Sons, Inc.