Beyond Pair Potentials in Elemental Transition Metals and Semiconductors

The chapter presents a review that describes the main approaches to transcending pair potential descriptions of transition metals and semiconductors. The chapter discusses the technologically important materials, because the desire to understand their materials properties has led to a large number of atomistic simulation studies, and, consequently, a great need for simplified energy functionals. At first glance, the bonding mechanisms in these two classes of materials seem to be quite distinct. One naively thinks of metallic atoms as closely packed hard balls with relatively weak attractive forces. In semiconductors, one generally focuses on strong bonds with charge accumulations between the atoms. The open structures of diamond structure semiconductors immediately suggest the presence of strong angular forces; if radial pair forces dominated, one would expect that the energy could be lowered by filling in the holes in the structure, thereby increasing the coordination number. In contrast, the closely packed structures of transition metals are at least consistent with a description based on radial forces. However, to obtain a more sophisticated picture of transition metals, including structural energy differences, the chapter mentions that angular forces are necessary, Furthermore, much of the understanding that has recently been gained of transition metal bonding is also applicable to semiconductors, and is being incorporated in the most recent semiconductor energy functionals.