Towards a microscopic description of the free-energy landscape of water

TL;DR

This paper develops a microscopic framework using configuration-space-networks and molecular dynamics to analyze water's free-energy landscape, revealing distinct organizational regimes across temperature ranges.

Contribution

It introduces a detailed microscopic description of water's free-energy landscape based on microstates and molecular simulations, linking structure and dynamics.

Findings

At ambient conditions, water exhibits multiple short-lived basins.

Below ambient temperature, the landscape becomes funnel-shaped and homogeneous.

Near the Widom line, the funnel steepens with limited microstate interconversions.

Abstract

Free-energy landscape theory is often used to describe complex molecular systems. Here, a microscopic description of water structure and dynamics based on configuration-space-networks and molecular dynamics simulations of the TIP4P/2005 model is applied to investigate the free-energy landscape of water. The latter is built on top of a large set of water microstates describing the kinetic stability of local hydrogen-bond arrangements up to the second solvation shell. In temperature space, the landscape displays three regions with an overall different organization. At ambient conditions, the free-energy surface is characterized by structural inhomogeneities with multiple, structurally well defined, short-lived basins of attraction. Below around ambient temperature, the liquid rapidly becomes homogeneous. In this regime, the landscape is funneled-like, with fully-coordinated water arrangements at the bottom of the funnel. Finally, a third region develops below the temperature of maximal compressibility (Widom line) where the funnel becomes steeper with few interconversions between microstates other than the fully coordinated ones. Our results present a viable a way to manage the complexity of water structure and dynamics, connecting microscopic properties to its ensemble behavior.