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arXiv2006

Entropy-based measure of structural order in water

We analyze the nature of the structural order established in liquid TIP4P water in the framework provided by the multi-particle correlation expansion of the statistical entropy. Different regimes are mapped onto the phase diagram of the model upon resolving the pair entropy into its translational and orientational components. These parameters are used to quantify the relative amounts of positional and angular order in a given thermodynamic state, thus allowing a structurally unbiased definition of low-density and high-density water. As a result, the structurally anomalous region within which both types of order are simultaneously disrupted by an increase of pressure at constant temperature is clearly identified through extensive molecular-dynamics simulations.

cond-mat.mtrl-sci↗

arXiv2005

High-pressure phase diagram of the exp-6 model: The case of Xenon

We investigated numerically the high-temperature/high-pressure phase diagram of Xenon as modelled through the exp-6 interaction potential, which is thought to provide a reliable description of the thermal behaviour 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 the 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 does never significantly differ from that of the FCC solid of equal T and P, though the former is found to slightly overcome 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.

cond-mat.mtrl-sci↗