Understanding water's anomalies with locally favored structures

TL;DR

This paper introduces a new structural order parameter based on microscopic data to explain water's anomalies, accurately modeling its phase behavior and revealing the role of locally favored structures with five-membered rings.

Contribution

It presents a novel order parameter and a two-state model that quantitatively describe water's anomalies from molecular arrangements.

Findings

The order parameter effectively characterizes water's state.

The model predicts density and compressibility anomalies.

Locally favored structures contain five-membered rings, influencing crystallization.

Abstract

Water is a complex structured liquid of hydrogen-bonded molecules that displays a surprising array of unusual properties, also known as water anomalies, the most famous being the density maximum at about $4^\circ$C. The origin of these anomalies is still a matter of debate, and so far a quantitative description of water's phase behavior starting from the molecular arrangements is still missing. Here we provide a simple physical description from microscopic data obtained through computer simulations. We introduce a novel structural order parameter, which quantifies the degree of translational order of the second shell, and show that this parameter alone, which measures the amount of locally favored structures, accurately characterizes the state of water. A two-state modeling of these microscopic structures is used to describe the behavior of liquid water over a wide region of the phase diagram, correctly identifying the density and compressibility anomalies, and being compatible with the existence of a second critical point in the deeply supercooled region. Furthermore, we reveal that locally favored structures in water not only have translational order in the second shell, but also contain five-membered rings of hydrogen-bonded molecules. This suggests their mixed character: the former helps crystallization, whereas the latter causes frustration against crystallization.