Core-softened potentials and the anomalous properties of water

TL;DR

This paper investigates a simplified particle model with core-softened potentials that replicates many of water's anomalous thermodynamic properties, including density maxima and multiple crystalline phases.

Contribution

It introduces a model with a hard core and linear repulsive shoulder that qualitatively explains water's anomalies and extends to include attractive forces for a comprehensive phase diagram.

Findings

The model exhibits density and compressibility maxima similar to water.

Multiple stable crystalline structures are predicted by the phase diagram.

Inclusion of attraction yields a liquid-gas critical point and metastable fluid line.

Abstract

We study the phase diagram of a system of spherical particles interacting in three dimensions through a potential consisting of a strict hard core plus a linear repulsive shoulder at larger distances. The phase diagram (obtained numerically, and analytically in a limiting case) shows anomalous properties that are similar to those observed in water. Specifically, we find maxima of density and isothermal compressibility as a function of temperature, melting with volume contraction, and multiple stable crystalline structures. If in addition a long range attraction between the particles is included, the usual liquid-gas coexistence curve with its critical point is obtained. But more interestingly, a first order line in the metastable fluid branch of the phase diagram appears, ending in a new critical point, as it was suggested to occur in water. In this way the model provides a comprehensive, consistent and unified picture of most of the anomalous thermodynamical properties of water, showing that all of them can be qualitatively explained by the existence of two competing equilibrium values for the interparticle distance.