Ab initio studies of silica-based clusters. Part II. Structures and energies of complex clusters

We report density functional calculations on the energies and conformations of complex silica-based clusters, after our analysis of the simpler cluste rs reported in part I. We report calculated structures, charge distribution s, and energies of the noncyclic four- and five-silicon chains, the branche d trimer and tetramer rings, the double trimer rings, the tetramer plus tri mer rings, the five- and six-silicon rings; all calculations are at the loc al density level of approximation. The total condensation energy (from the monomer) to form a silica cluster in the gas phase depends essentially on i ts structure and size. Our results show that the stability of the noncyclic clusters decreases with the degree of branching, as observed experimentall y. This trend is observed for both four-silicon and five-silicon clusters. As expected, the double ring clusters are quite unstable, which is especial ly marked for the double trimer rings. Formation of trimer-tetramer rings i s energetically more favorable, particularly for the edge-linked cluster. T he four- and six-silicon rings are more stable than the corresponding penta mer because of the relatively asymmetric arrangement of the latter.