A Unified Description of the Liquid Structure, Static and Dynamic Anomalies, and Criticality of TIP4P/2005 Water by a Hierarchical Two-State Model

Water has anomalous thermodynamic and kinetic properties distinct from ordinary liquids. The famous examples are the density maximum at 4 °C and the viscosity decrease upon pressurization. The presence of the second critical point has been considered to be responsible for these anomalies since its discovery in ST2 water. Recently, its existence has been confirmed firmly in TIP4P/2005, which is one of the most successful classical models of water, by Debenedetti et al. [Debenedetti et al. Science 2020, 369, 289]. Here, we study the water structure and thermodynamic and dynamic quantities in a wide temperature (T)-pressure (P) range, including the vicinity of the second critical point, by extensive molecular dynamics simulation of this water model. We reveal that a hierarchical two-state model with the cooperative formation of water tetrahedral structures via hydrogen bonding can describe the T, P-dependences of structure, thermodynamic and kinetic anomalies, and criticality of TIP4P/2005 water in a unified manner. TIP4P/2005 water shows very similar behaviors to real water in all these aspects, suggesting a possible existence of the second critical point in the water. Our physical description based on the two order parameters, the density and the fraction of locally favored tetrahedral structures, indicates that the latter is the relevant order parameter for the second critical point, which is supported by the analysis of the critical fluctuations. The different nature of the density and the fraction of tetrahedral arrangements, conserved and nonconserved, may be key to unambiguously identifying the relevant order parameter.