An interatomic potential has been developed to describe the dynamics of Si/Cl systems, with particular relevance to reactive ion etching of Si by energetic Cl ions. We have modified the Stillinger-Weber (SW) potential of Feil et al. by adding two new terms: (1) an embedding term that corrects for the variation in Si-Cl bond strength as a function of the number of neighbors, and (2) a four-body term to describe the variation of the Si-Si bond strength as a function of the number of neighbors of each Si atom and the atom types (a bond order correction). Calculated Si etch rates obtained from molecular dynamics simulations using the new potential are in better agreement with recent experimental results than those obtained with the unmodified potential. Predictions of the stoichiometry of the etch products are also markedly different between the two potentials.