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 lowerature "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.