Connection between water's dynamical and structural properties: insights from ab initio simulations

TL;DR

This study uses ab initio molecular dynamics to explore how water's structural and dynamical properties relate across temperatures, revealing insights into the validity of transport relations and the importance of long-range structural features.

Contribution

It provides a detailed analysis of water's transport properties and structure using ab initio simulations, highlighting the role of different functionals and quantum effects in supercooled water.

Findings

SCAN functional better matches experimental viscosity and diffusion

Nuclear quantum effects influence water dynamics at low temperatures

Excess entropy scaling relations hold across different functionals

Abstract

Among all fluids, water has always been of special concern for scientists from a broad variety of research fields due to its rich behavior. In particular, some questions remain unanswered nowadays concerning the temperature dependence of bulk and interfacial transport properties of supercooled and liquid water, e.g. regarding the fundamentals of the violation of the Stokes-Einstein relation in the supercooled regime or the subtle relation between structure and dynamical properties. Here we investigated the temperature dependence of the bulk transport properties from ab initio molecular dynamics based on density functional theory, down to the supercooled regime. We determined from a selection of functionals, that SCAN better describes the experimental viscosity and self-diffusion coefficient, although we found disagreements at the lowest temperatures. For a limited set of temperatures, we also explored the role of nuclear quantum effects on water dynamics using ab initio molecular dynamics that has been accelerated via a recently introduced machine learning approach. We then investigated the molecular mechanisms underlying the different functionals performance and assessed the validity of the Stokes-Einstein relation. We also explored the connection between structural properties and the transport coefficients, verifying the validity of the excess entropy scaling relations for all the functionals. These results pave the way to predict the transport coefficients from the radial distribution function, helping to develop better functionals. On this line, they indicate the importance of describing the long-range features of the radial distribution function.