Study of the ST2 model of water close to the liquid-liquid critical point

TL;DR

This study uses advanced Monte Carlo simulations to investigate the ST2 water model near its hypothesized liquid-liquid critical point, confirming previous findings and demonstrating the robustness of the critical behavior independent of electrostatic interaction treatment.

Contribution

The paper provides evidence supporting the existence and location of the liquid-liquid critical point in the ST2 water model, validating previous results with different electrostatic interaction methods.

Findings

Supports previous evidence for the critical point

Shows liquid equilibrates without crystal formation

Confirms bimodal density as a liquid-state phenomenon

Abstract

We perform successive umbrella sampling grand canonical Monte Carlo computer simulations of the original ST2 model of water in the vicinity of the proposed liquid-liquid critical point, at temperatures above and below the critical temperature. Our results support the previous work of Y. Liu, A.Z. Panagiotopoulos and P.G. Debenedetti [J. Chem. Phys. {\bf 131}, 104508 (2009)], who provided evidence for the existence and location of the critical point for ST2 using the Ewald method to evaluate the long-range forces. Our results therefore demonstrate the robustness of the evidence for critical behavior with respect to the treatment of the electrostatic interactions. In addition, we verify that the liquid is equilibrated at all densities on the Monte Carlo time scale of our simulations, and also that there is no indication of crystal formation during our runs. These findings demonstrate that the processes of liquid-state relaxation and crystal nucleation are well separated in time. Therefore, the bimodal shape of the density of states, and hence the critical point itself, is a purely liquid-state phenomenon that is distinct from the crystal-liquid transition.