PHASE BEHAVIOR OF SIMULATED METASTABLE WATER: A NEW THERMODYNAMIC SCENARIO

Abstract

The thermodynamic properties of commonly used water pair-potentials (ST2, TIP4P, and SPC/E) are calculated from molecular dynamics simulations. In particular, the properties of supercooled and stretched states are found, yielding a determination of the equation of state (EOS) in this region for the metastable liquid. The ability of these models to reproduce the unique thermodynamic properties of water is confirmed. The calculated EOS does not exhibit a re-entrant liquid spinodal and also predicts that the line of density maxima in the phase diagram has a maximum in temperature <i>T</i> as a function of pressure <i>P</i>. In addition, an unexpected feature appears at lower <i>T</i>, in the form of an inflection in the liquid phase isotherms of <i>P</i> versus density &rho;, suggesting that a critical point occurs in the liquid state phase diagram at low <i>T</i>. This critical point may be the end-point of a line of first-order phase transitions, separating two liquid phases of different density. We propose to relate these two liquid phases to the low and high density amorphous ices.