A computational study of excess properties for mW potential model of water in supercooled region

In this work, we studied the liquid–crystal transition of water using monatomic potential model (mW) (Molinero and Moore, 2009). To simulate water, the NPT-MC simulation were performed and obtained the average properties as potential energy (〈ϕ′〉) and density (〈ρ〉) at various temperatures which are in the very good agreement (i.e. 〈ϕ′〉=−1.766 and 〈ρ〉=0.448 at 202 K) with the literature values (Hujo et al., 2011). The free energy difference between the two liquid phases of water was computed by thermodynamic integration along with a reversible path at zero pressure and various temperatures. The recently developed thermodynamic integration (TDI) method was used to computed the excess Gibbs free energy of high density liquid (HDL) phase with respect to the crystalline phase at different temperatures in the supercooled region of mW-water model (Molinero and Moore, 2009). Based on the slope of excess Gibbs free energy with respect to temperature the excess entropy of the high density liquid (HDL) phase was obtained, which shows an anomalous behavior at or near the liquid–liquid transition temperature of Tll=202 K (Molinero and Moore, 2009). The excess entropy of the liquid phase in the supercooled mW water shows a sharp decline at or near the liquid–liquid phase transition temperature (Tll). The excess entropy of the HDL phase decreases as the temperature decreases to 197 K and increases at 192 K. Our results are in good agreement with the previous observation of a anomalous dependency of the density on temperature in MD simulations starting in the HDL phase at a temperature just above Tll (Moore and Molinero, 2009).