Lattice Gas with Short-Range Pair Interactions and a Singular Coexistence-Curve Diameter

A simple lattice gas, of the familiar kind with only short-range pair interactions, is proved to have a coexistence curve whose diameter is singular at the critical point. Particle-hole symmetry is violated only because the range of interaction energies a molecule can experience depends on which of two nonequivalent sets its lattice site belongs to.