How van der Waals interactions determine the unique properties of water

TL;DR

This paper demonstrates that van der Waals interactions are essential for explaining water's unique properties, such as its density maximum and negative melting volume, using efficient neural network potentials derived from ab initio simulations.

Contribution

It introduces neural network potentials that enable systematic study of vdW forces in water, revealing their crucial role in water's anomalous behaviors.

Findings

vdW interactions are key to water's density maximum

vdW forces influence the negative volume of melting

flexibility of hydrogen bond network is affected by vdW forces

Abstract

While the interactions between water molecules are dominated by strongly directional hydrogen bonds (HBs), it was recently proposed that relatively weak, isotropic van der Waals (vdW) forces are essential for understanding the properties of liquid water and ice. This insight was derived from ab initio computer simulations, which provide an unbiased description of water at the atomic level and yield information on the underlying molecular forces. However, the high computational cost of such simulations prevents the systematic investigation of the influence of vdW forces on the thermodynamic anomalies of water. Here we develop efficient ab initio-quality neural network potentials and use them to demonstrate that vdW interactions are crucial for the formation of water's density maximum and its negative volume of melting. Both phenomena can be explained by the flexibility of the HB network, which is the result of a delicate balance of weak vdW forces, causing e.g. a pronounced expansion of the second solvation shell upon cooling that induces the density maximum.