Free energy surface of ST2 water near the liquid-liquid phase transition

TL;DR

This study uses umbrella sampling Monte Carlo simulations to explore the free energy landscape of the ST2 water model near its hypothesized liquid-liquid critical point, revealing two distinct liquid phases distinguished by density.

Contribution

It provides the first detailed free energy surface analysis of the ST2 water model near the liquid-liquid critical point, identifying two liquid basins with different densities.

Findings

Identification of two basins with liquid-like bond order but different densities

Consistency of free energy surface shape with low and high density liquid phases

Pressure and temperature dependence supports phase distinction

Abstract

We carry out umbrella sampling Monte Carlo simulations to evaluate the free energy surface of the ST2 model of water as a function two order parameters, the density and a bond-orientational order parameter. We approximate the long-range electrostatic interactions of the ST2 model using the reaction-field method. We focus on state points in the vicinity of the liquid-liquid critical point proposed for this model in earlier work. At temperatures below the predicted critical temperature we find two basins in the free energy surface, both of which have liquid-like bond orientational order, but differing in density. The pressure and temperature dependence of the shape of the free energy surface is consistent with the assignment of these two basins to the distinct low density and high density liquid phases previously predicted to occur in ST2 water.