Statistical Mechanical Theory of Liquid Water

Water is an unusual liquid. Its thermophysical properties are nonmonotonic with temperature T and pressure p. It is not known how water’s behaviors are encoded in its molecules. We give a statistical mechanical model, Cage Water, which assumes three bonding states: van der Waals, pairwise hydrogen bonding, and multibody cooperative caging hydrogen bonds. The model is analytical and so very fast to compute. It gives properties of liquid water in excellent agreement with extensive pT experiments, as good as those of explicit water models TIP4P/2005 and MB-pol at a much lower cost. Through readily interpretable substates, Cage Water explains water’s liquid anomalies, including its controversial liquid–liquid supercooling transition, as simple switchovers among the three bonding types.