Liquid-Liquid Criticality from First Principles in a Flexible Model of Water

A metastable liquid-liquid critical point is demonstrated and rigorously located under deeply supercooled conditions in the WAIL model of water, a classical force field parameterized using only ab initio data only as opposed to specific thermophysical properties. The hypothesis that the anomalous behavior of liquid water is related to the existence of a second critical point in deeply supercooled states has long been the subject of intense debate. Recent experimental results and simulations studies based on rigid classical models parameterized to reproduce thermodynamic properties of water have provided support to this hypothesis. The WAIL model incorporates several novel features: it is both flexible and polarizable, properties which can significantly influence the phase behavior of water. The relationship between the second critical point and the two distinct liquids which form in deeply supercooled states with water thermodynamic anomalies is confirmed by the present study in which the water-water phase separation is predicted using only ab-initio calculations. The manner in which the presence of two distinct but interconvertible structures give rise to the well-known thermophysical anomalies of deeply supercooled water is also shown empirically.