In a critical evaluation, we show that existing classical potentials are not suitable for calculating the energy of realistic atomic processes in Si. We present a new potential which is especially suited to simulate processes in the diamond lattice rather than in high-energy bulk structures of Si. Our potential is based on a very large quantum-mechanical data base. It consists of two- and three-body terms with short-range separable forms, and reproduces accurately the energy surface for atomic exchange in Si. Thus, it is ideally suited for molecular dynamics simulations of atomic processes in Si. © 1988 The American Physical Society.