Charge fluctuations and correlation strength in chemical bonds: First-row homonuclear diatomic molecules

Abstract

We investigate, by means of ab initio calculations, the strength of electron correlations within covalent bonds: the first-row homonuclear diatomics serve as test cases. As an appropriate measure of the correlation strength, we introduce the reduction of the mean-square deviations of the electronic charges in localized orbitals forming a bond. A recently developed population analysis in terms of local operators derived from localized molecular orbitals is thereby used. The correlation-strength parameter depends only weakly on dynamical correlations as test calculations demonstrate. Therefore, the full-valence complete active space self-consistent field (CASSCF) approximation is applied in order to study the changes in the correlation strength with changing bond length for different types of bonds. A number of simple rules emerge from this discussion. In addition, we show that charge fluctuations are not only a reliable measure of intrabond correlation effects, but also can be used to monitor intraatomic quasi-degeneracy effects as well as the interdependence within multiple bonds. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 67: 157–173, 1998