An ab initio study of the structure of two-, three- and five-dimer silicon clusters: An approach to the Si(100) surface

Abstract

Two-, three- and five-dimer cluster models were used to elucidate the structures of small models of the Si(100) surface. Because surface dimers have been shown to have significant singlet diradical character, multi-reference wave functions were used in order to obtain a reliable description of such species. CASSCF (complete active space SCF) geometry optimizations find symmetric structures to be the global minima, with no local minima at buckled structures. This result for the three- and five-dimer clusters implies that dimer- dimer repulsions are not sufficient to cause buckling of these species. These results are in contrast with most previous calculations that predict buckling of surface dimers. The effect of the dynamic part of the electron correlation on surface structure was assessed by performing single point multi-reference perturbation theory (MRMP) calculations along the three buckling normal modes of the three-dimer cluster. Although dynamic correlation effects are found to "soften" motion along the buckling coordinates, the surface remains unbuckled when such correlation effects are included. The MRMP results are in qualitative agreement with the CASSCF predictions. The implications of these results with regard to the structure of the Si(100) surface are discussed.