Water's two-critical-point scenario in the Ising paradigm.

We present a spin-1, three-state Ising model for the unusual thermodynamics of fluid water. Thus, besides vacant cells, we consider singly occupied cells with two accessible volumes in such a way that the local structures of low density, energy, and entropy associated with water's low-temperature "icelike" order are characterized. The model has two order parameters that drive two phase transitions akin to the standard gas-liquid transition and water's hypothesized liquid-liquid transition. Its mean-field equation of state enables a satisfactory description of results from experiments and simulations for the ST2 and TIP4P/2005 force fields, from the phase diagram, the density maximum, or the deeply "stretched" states to the behavior of thermodynamic response functions at low temperatures at which water exists as a supercooled liquid. It is concluded that the model may be regarded as a most basic prototype of the so-called "two-critical-point scenario."