Entropy-based measure of structural order in water

TL;DR

This paper introduces an entropy-based method to quantify and distinguish between positional and angular order in liquid water, revealing the structurally anomalous region where both orders diminish with pressure increase.

Contribution

It provides a novel entropy-based framework to analyze structural order in water, mapping different regimes onto the phase diagram and identifying the anomalous region.

Findings

Identified the region where both positional and angular order decrease with pressure.

Quantified the relative contributions of translational and orientational order in water.

Mapped structural regimes onto the phase diagram using entropy components.

Abstract

We analyze the nature of the structural order established in liquid TIP4P water in the framework provided by the multi-particle correlation expansion of the statistical entropy. Different regimes are mapped onto the phase diagram of the model upon resolving the pair entropy into its translational and orientational components. These parameters are used to quantify the relative amounts of positional and angular order in a given thermodynamic state, thus allowing a structurally unbiased definition of low-density and high-density water. As a result, the structurally anomalous region within which both types of order are simultaneously disrupted by an increase of pressure at constant temperature is clearly identified through extensive molecular-dynamics simulations.