Liquid-Liquid Critical Point in 3D Many-Body Water Model

Many-body interactions can play a relevant role in water properties. Here we study by Monte Carlo simulations a coarse-grained model for bulk water that includes many-body interactions associated to water cooperativity. The model is efficient and allows us to equilibrate water at extreme low temperatures in a wide range of pressures. We find the line of temperatures of maximum density at constant pressure and, at low temperature and high pressure, a phase transition between high-density liquid and low-density liquid phases. The liquid-liquid phase transition ends in a critical point. In the supercritical liquid region we find for each thermodynamic response function a locus of weak maxima at high temperature and a locus of strong maxima at low temperature, with both loci converging at the liquid-liquid critical point where they diverge. Comparing our results with previous works for the phase diagram of a many-body water monolayer, we observe that the weak maxima of the specific heat are less evident in bulk and appear only at negative pressures, while we find those of compressibility and thermal expansion coefficient also at positive pressures in bulk. However, the strong maxima of compressibility and thermal expansion coefficient are very sharp for the bulk case. Our results clarify fundamental properties of bulk water, possibly difficult to detect with atomistic models not accounting for many-body interactions.