THE NATURE OF THE COVALENT BOND - THE EXISTENCE AND ORIGIN OF NONNUCLEAR ATTRACTORS

It has generally been assumed that the only maxima of the electron density distribution in any molecular system occur at the positions of the nuclei, with computational data and theoretical considerations being in full accord. However, in the charge distributions of a few diatomic molecules and multinuclear clusters, maxima at the midpoint between the nuclei (nonnuclear attractors) have recently been shown to exist. In this paper, we present full CI calculations on Li2 to confirm that the nonnuclear attractors are not an artifact of the level of quantum chemical calculations and offer a theoretical explanation of the origin of the effect. Within our approach, the midpoint maxima result from the coupling of charge distributions in the ground and appropriate excited electronic states induced by electron-electron interactions. Such coupling may cause instability of the ground state electron density at the midpoint between the nuclei to produce the midbond maximum. Parameters of the electronic subsystem which determine the effect are discussed, in particular for the case of homonuclear diatomic molecules.