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Valence-bond model for silicon force fields.

We propose a new type of classical force-field model for silicon in which the form is based on a valence-bond approach. The results of quantum-mechanical calculations on small clusters, together with bulk experimental data are used to obtain the parameters of the force field. The importance of including a description of bent and long bonds in the potential is discussed. It is shown that a description of such bonds not only accounts for the general compactness of cluster geometries found from electronic structure calculations but also provides reasonable surface reconstructions. Details of cluster geometries are not accounted for in the present potential because a number of effects found in the quantum-mechanical calculations are not included in the present force-field model. Likewise, effects important for describing the high-pressure solid-state phases of silicon are also not included in the present potential. The approach used here is to incorporate very specific and well controlled effects in the potential and determine their consequences for clusters and surfaces.