Three-phase equation of state by the lattice gas model

A rigid lattice gas model of solid-liquid, liquid-gas, and solid-gas transformations is proposed. The total free energy is separated into a lattice part and a vibrational correction part. It is reported that the vibrational part plays an important role in obtaining the gas, liquid and solid phases and the correct phase transitions. A method of combining the two lowest order approximations (single site, two-site) in the cluster variation method is suggested in describing the lattice free energy. The critical point and the triple point of argon are reproduced fairly well. Results are compared with the expandable lattice theory demonstrating the appropriateness of the present model in the vicinity of the triple point.