Atomic dynamics in fluids: Normal mode analysis revisited

TL;DR

This paper investigates the nature of atomic normal modes in liquids, revealing they interpolate between solid-like vibrational modes and gas-like collisional modes as temperature increases, challenging traditional views.

Contribution

It introduces a new interpretation of normal modes in liquids as a blend of solid-like and gas-like behaviors based on eigenmode analysis at various temperatures.

Findings

Normal modes become more collisional and translational with temperature.

Normal modes interpolate between solid and gas descriptions.

Normal modes in liquids can be described by both solid-like and gas-like modes.

Abstract

Developing microscopic understanding of the thermal properties of liquids is challenging due to their strong dynamic disorder, which prevents characterization of the atomic degrees of freedom. There have been significant research interests in the past few decades to extend the normal mode analysis for solids to instantaneous structures of liquids. However, the nature of normal modes that arise from these unstable structures is still elusive. In this work, we explore the instantaneous eigenmodes of dynamical matrices of various Lennard-Jones argon liquid/gas systems at high temperatures and show that the normal modes can be interpreted as an interpolation of T \to \infty (gas) and T = 0 (solid) mode descriptions. We find that normal modes become increasingly collisional and translational, recovering atomistic gas-like behavior rather than vibrational with increase in temperature, suggesting that normal modes in liquids may be described by both solid-like and gas-like modes.