Effects of translational and rotational degrees of freedom on the properties of model water

TL;DR

This study uses molecular dynamics simulations to explore how separate control of rotational and translational temperatures affects water's structure, hydrogen bonding, and diffusion, revealing distinct impacts of each degree of freedom.

Contribution

It provides new insights into how rotational and translational degrees of freedom differently influence water's properties at a molecular level.

Findings

Increasing rotational temperature breaks hydrogen bonds significantly.

Higher rotational temperature reduces the probability of large cavities in water.

Diffusion coefficient shows non-monotonous behavior with rotational temperature.

Abstract

Molecular dynamics simulations with separate thermostats for rotational and translational motions were used to study the effects of these degrees of freedom on the structure of water at a fixed density. To describe water molecules, we used the SPC/E model. The results indicate that an increase of the rotational temperature, $T_\textrm{R}$, causes a significant breaking of the hydrogen bonds. This is not the case, at least not to such an extent, when the translational temperature, $T_\textrm{T}$, is raised. The probability of finding an empty spherical cavity (no water molecule present) of a given size, strongly decreases with an increase of $T_\textrm{R}$, but this only marginally affects the free energy of the hydrophobe insertion. The excess internal energy increases proportionally with an increase of $T_\textrm{R}$, while an increase of $T_\textrm{T}$ yields a much smaller effect at high temperatures. The diffusion coefficient of water exhibits a non-monotonous behaviour with an increase of the rotational temperature.