In this study, we use ab initio molecular modeling and a cluster expansion technique to derive interaction potentials for sulfur and selenium based on fundamental physics. We consider both elemental sulfur and selenium and binary compounds thereof. The resulting interaction potentials consist of two-body, three-body, and effective four-body terms. We use these potentials in classical Monte Carlo simulations to investigate the structure of S xSe 1-x glasses. Calculated structural data include radial distribution functions, bond distributions, and chain/ring length analyses. We find that S xSe 1-x glasses consist of long two-coordinated chains and rings of atoms. The glasses exhibit both homopolar and heteropolar bonding, with homopolar bonding somewhat preferable at lower sulfur content (x < 0.7).