Electron correlations in molecules. I Bond orbital approximation

Abstract

A simple and fairly accurate approach to electron correlations in the ground state of molecules is presented. The two parts of electron correlations, inter- and intra-atomic correlations, are treated separately and by different methods. The interatomic correlations are studied by means of a variational local ansatz. It starts from the self-consistent field (SCF) ground state and optimizes the energy by the appropriate reduction of charge fluctuations. This procedure is qualitatively illustrated in the bond orbital approximation (BOA). It is found that the correlation energy decreases with increasing bond polarity α p, being proportional to the factor (1 – α 2 p) 5 2 . It is also shown that the contributions to the correlation energy due to one-particle excitations become important only in more strongly correlated polar bonds. The intraatomic correlations are obtained from an atoms-in-molecule-type of approach. It makes use of a population analysis of the correlated ground state wavefunction. The used probability distribution P( n), where n is the valence electron number, is found to be well approximated by a normal distribution. The advantages and limitations of the method are shortly discussed.