Electron correlations in hydrocarbon molecules

An extensive investigation is made of electron correlations in the ground state of hydrocarbon molecules. This is done by starting from a semiempirical self-consistent field (SCF) calculation. The interatomic correlations are studied by means of a previously developed local approach to the correlation problem. It is demonstrated that the various contributions to the interatomic correlation energy can be described by simple analytical expressions which depend on the type of bond as well as on the bond lengths and angles. The results are easily understood in physical terms. The intraatomic correlations are obtained from an atoms in molecules type of approach as elaborated by Lievin et al. It makes use of a population analysis of the ground-state wave function as well as of calculations for the C and H atoms. We have then a very simple, and as it turns out rather accurate calculational scheme at hand. The computational times involved are of the same order as those for the fast SCF programs. As an application of the theory we have calculated the correlation energy difference between naphthalene and azulene.Dedicated to B. Mühlschlegel on the occasion of his 60th birthday