An ab initio intermolecular pair potential for hydrogen chloride has been computed using symmetry-adapted perturbation theory and an extended basis set at a level of theory equivalent to fourth-order Moller-Plesset perturbation theory. Three different site-site pair potential functions were used to fit the ab initio energies, and these were then used to calculate the second virial coefficient and in Gibbs ensemble Monte Carlo simulations to determine the vapor-liquid equilibria. The accurate predictions of the phase behavior compared to experimental data with only pairwise interactions suggest that these are the predominant contribution, and that nonpairwise additivity and quantum effects are not important for HCl. Our results are also compared with those of another ab initio-based pair potential previously reported in the literature. (C) 2003 American Institute of Physics.