Tuning the liquid-liquid transition by modulating the hydrogen bond angular flexibility in a model for water

TL;DR

This study extends the ST2 water model to explore how hydrogen bond angular flexibility influences the liquid-liquid transition, demonstrating that increased flexibility shifts the critical point to more experimentally accessible conditions.

Contribution

It introduces a continuous modification to the ST2 model, revealing the impact of bond flexibility on the thermodynamic stability and liquid-liquid transition in water.

Findings

Increased bond flexibility moves the liquid-liquid critical point to higher temperatures.

The liquid-liquid transition is confirmed as a genuine phenomenon in the model.

Flexibility affects the relative stability between liquid water and ice.

Abstract

We propose a simple extension of the well known ST2 model for water [F.H. Stillinger and A. Rahman, J. Chem. Phys. {\bf 60}, 1545 (1974)] that allows for a continuous modification of the hydrogen bond angular flexibility. We show that the bond flexibility affects the relative thermodynamic stability of the liquid and of the hexagonal (or cubic) ice. On increasing flexibility, the liquid-liquid critical point, which in the original ST2 model is located in the no-man's land (i. e. the region where ice is the thermodynamically stable phase) progressively moves to a temperature where the liquid is more stable than ice. Our study definitively proves that the liquid-liquid transition in ST2 is a genuine phenomenon, of high relevance in all tetrahedral network-forming liquids, including water.