The conflict between hole delocalisation and static plus dynamic polarisation in molecular cations: illustration on Mg n + clusters

Abstract

In a filled band problem, like an alkaline earth or a rare gas cluster, the positive ion is stabilised by both the hole delocalisation and by polarisation effects. The variational monoelectronic pictures take into account the first factor and the static polarisation, disregarding the dynamic or instantaneous polarisation, which is a correlation effect. This unbalance of ΔSCF treatments underestimates the binding energy and unduly favors the structures in which the hole is concentrated on an atom surrounded by numerous neighbors. Taking into account the dynamic polarisation through CI or effective VB approaches restores the relative stability of structures where the hole is spread over a large number of atoms, as illustrated on the Mg 4 + problem. This remark is shown to have general implications for covalent molecules.