Ab initio van der Waals interactions in simulations of water alter structure from mainly tetrahedral to high-density-like

TL;DR

This study uses ab initio molecular dynamics with van der Waals density functionals to show that including vdW interactions shifts water's structure from tetrahedral to high-density-like, aligning with a two-liquid model.

Contribution

It demonstrates how vdW interactions in density-functional theory alter water's microscopic structure, bridging simulation results with experimental two-liquid models.

Findings

Inclusion of vdW interactions softens water's structure.

The O-O pair-correlation function shifts towards high-density water characteristics.

A linear combination of simulated and experimental data reproduces ambient water structure.

Abstract

The structure of liquid water at ambient conditions is studied in ab initio molecular dynamics simulations using van der Waals (vdW) density-functional theory, i.e. using the new exchange-correlation functionals optPBE-vdW and vdW-DF2. Inclusion of the more isotropic vdW interactions counteracts highly directional hydrogen-bonds, which are enhanced by standard functionals. This brings about a softening of the microscopic structure of water, as seen from the broadening of angular distribution functions and, in particular, from the much lower and broader first peak in the oxygen-oxygen pair-correlation function (PCF), indicating loss of structure in the outer solvation shells. In combination with softer non-local correlation terms, as in the new parameterization of vdW-DF, inclusion of vdW interactions is shown to shift the balance of resulting structures from open tetrahedral to more close-packed. The resulting O-O PCF shows some resemblance with experiment for high-density water (A. K. Soper and M. A. Ricci, Phys. Rev. Lett., 84:2881, 2000), but not directly with experiment for ambient water. However, an O-O PCF consisting of a linear combination of 70% from vdW-DF2 and 30% from experiment on low-density liquid water reproduces near-quantitatively the experimental O-O PCF for ambient water, indicating consistency with a two-liquid model with fluctuations between high- and low-density regions.