The mixed ionocovalent bonding in small insulating clusters

We report a selfconsistent tight binding calculation of the electronic and structural properties of small insulating clusters (up to 12 atoms): these include alkaline-earth oxides (MgO and CaO) and NaCl clusters. Interatomic distance contractions are found in most cases, as in small metallic clusters. In addition, new effects take place in some of the compounds; these are a reduction of the ionic charge and, for the first time, the prediction of a dimerization of the interatomic distances. The microscopic processes responsible for their occurence and size dependence are dicussed with analytical arguments. It is shown that for these small clusters the equilibrium configuration and the electronic structure are fixed by a competition between covalent and electrostatic interactions, especially in the oxides.