The temperaturehyphen;dependent phase equilibrium of simple classical molecules has been studied, using a model based on the twohyphen;dimensional square lattice. The intermolecular potential includes a hard core extending to the firsthyphen;neighbor distance and a finite interaction (attractive or repulsive) at the secondhyphen;neighbor distance. The transfer matrix method of calculation is used for lattices of infinite length and finite circumference up to 16 sites. At all temperatures studied there is a single phase transition, which is of first order for attractive interactions at sufficiently low temperatures. The model best describes the equilibrium between solid and gas at low temperatures or the equilibrium between solid and supercritical fluid at high temperatures. The deficiencies in the model which exclude a liquidlike phase are discussed.
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