High pressure phase diagram of an exp-6 fluid: The case of Xe

We investigated numerically the high-temperature–high-pressure phase diagram of xenon as modeled through the exp-6 interaction potential, which is thought to provide a reliable description of the thermal behavior of rare gases under extreme conditions. We performed a series of extensive NVT Monte Carlo simulations which, in conjunction with exact computation of the solid free energy by the Frenkel-Ladd method, allowed us to precisely locate the freezing and melting thresholds at each temperature. We find that, under isothermal compression, the exp-6 fluid freezes directly into a fcc solid; however, above 4500 K, an intermediate bcc phase becomes stable in a narrow range of pressures. The chemical potential of the hcp phase never significantly differs from that of the fcc solid of equal T and P, though the former is found to be slightly greater than the latter. We discuss our results in the light of previous numerical studies of the same model system and of the experimental data available for xenon.