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 clusters reported in part I. We report calculated structures, charge distributions, and energies of the noncyclic four- and five-silicon chains, the branched trimer and tetramer rings, the double trimer rings, the tetramer plus trimer rings, the five- and six-silicon rings; all calculations are at the local density level of approximation. The total condensation energy (from the monomer) to form a silica cluster in the gas phase depends essentially on its structure and size. Our results show that the stability of the noncyclic clusters decreases with the degree of branching, as observed experimentally. This trend is observed for both four-silicon and five-silicon clusters. As expected, the double ring clusters are quite unstable, which is especially marked for the double trimer rings. Formation of trimer-tetramer rings is energetically more favorable, particularly for the edge-linked cluster. The four- and six-silicon rings are more stable than the corresponding pentamer because of the relatively asymmetric arrangement of the latter.