Structural changes induced by an excess electron in small mercury clusters

We discuss structural differences between neutral and negatively charged me rcury clusters, with cluster sizes ranging from Hg-3 to Hg-7. We have optim ized various types of structures for both species using quantum chemical nb initio methods. Because of the binding character of the p-type orbital occ upied by the excess electron we observed a contraction of the bond length o f approximately 0.2 Angstrom on average. This corresponds to changes of the binding energy of the excess electron of 0.1 to 0.4 eV. In addition to thi s we encountered changes in the relative stability for different structures of the same cluster size by attaching an excess electron. The consequences for a meaningful comparison between theory and experiment are discussed in terms of recent measurements of electron affinities for mercury clusters. Finally, we propose a hybrid model for the incorporation of electron correl ation effects. It consists of pairwise additive effective dispersion potent ials and configuration interaction calculations that take into account corr elation contributions originating from the excess electron. Numerical tests have been performed for selected clusters and revealed good agreement with more accurate ab initio methods. (C) 2000 Elsevier Science B.V.