Electron correlations in molecules. II. General trends derived for isoelectronic series

We investigate general features of electronic correlations in four series of small isoelectronic molecules (partially model systems) containing hydrogen and first-row atoms. The calculations are done by combining a semi-empirical INDO scheme for the SCF part with the local ansatz for the calculation of interatomic correlations and an atoms-in-molecule approach for the determination of intraatomic correlation energies. The latter ones turn out to depend in the simple way on the mean number of electrons and the hybridization of an atom, but only weakly on the nuclear charge. We discuss how the individual (intra- and inter-bond) contributions to the interatomic correlation energy depend on the bond polarity, type of bonding and different kinds of atoms involved. Furthermore we characterize the suppression of charge fluctuations by interatomic correlations by means of a correlation strength parameter.