Local Structure Analysis in $Ab$ $Initio$ Liquid Water

TL;DR

This study uses the local structure index (LSI) to analyze the local environments in ab initio liquid water, revealing signatures of low- and high-density amorphous ice phases and their transient nature at ambient conditions.

Contribution

It demonstrates the presence of low- and high-density phase signatures in ambient liquid water's inherent structures using LSI analysis within an accurate DFT framework.

Findings

Most water molecules have disordered high-density-like environments at ambient conditions.

Inherent structures show a bimodal LSI distribution with a 3:1 ratio of high- to low-density-like molecules.

The persistence time of local environments is approximately 4 ps, indicating transient phase fluctuations.

Abstract

Within the framework of density functional theory, the inclusion of exact exchange and non-local van der Waals/dispersion (vdW) interactions is crucial for predicting a microscopic structure of ambient liquid water that quantitatively agrees with experiment. In this work, we have used the local structure index (LSI) order parameter to analyze the local structure in such highly accurate $ab$ $initio$ liquid water. At ambient conditions, the LSI probability distribution, P($I$), was unimodal with most water molecules characterized by more disordered high-density-like local environments. With thermal excitations removed, the resultant bimodal P($I$) in the inherent potential energy surface (IPES) exhibited a 3:1 ratio between high- and low-density-like molecules, with the latter forming small connected clusters amid the predominant population. By considering the spatial correlations and hydrogen bond network topologies $among$ water molecules with the same LSI identities, we demonstrate that the signatures of the experimentally observed low- (LDA) and high-density (HDA) amorphous phases of ice are present in the IPES of ambient liquid water. Analysis of the LSI autocorrelation function uncovered a persistence time of $\sim$ 4 ps---a finding consistent with the fact that natural thermal fluctuations are responsible for transitions between these distinct yet transient local aqueous environments in ambient liquid water.