Advanced Post-Processing Techniques of Molecular Dynamics Simulations in Studying Strong Anharmonic Thermodynamics of Solids

TL;DR

This paper discusses advanced post-processing methods for molecular dynamics simulations to better understand strong anharmonic thermodynamics in solids, revealing phenomena that harmonic models cannot capture.

Contribution

It introduces novel computational techniques to extract anharmonic vibrational information from molecular dynamics data, improving understanding of complex thermal behaviors in materials.

Findings

Unveiled negative thermal expansion in cuprite.

Explained high-temperature stability of rutile.

Demonstrated limitations of harmonic models at high temperatures.

Abstract

While the vibrational thermodynamics of materials with small anharmonicity at low temperatures has been understood well based on the harmonic phonons approximation; at high temperatures, this understanding must accommodate how phonons interact with other phonons or with other excitations. We shall see that the phonon-phonon interactions give rise to interesting coupling problems, and essentially modify the equilibrium and non-equilibrium properties of materials, e.g., thermal expansion, thermodynamic stability, heat capacity, optical properties, thermal transport and other nonlinear properties of materials. To date the anharmonic lattice dynamics is poorly understood despite its great importance, and most studies on lattice dynamics still rely on the harmonic or quasiharmonic models. With recent developement of computational models, the anharmonic information can be extracted from the atomic trajectories of molecular dynamics simulations. For example, the vibrational energy spectra, the effective potential energy surface and the phonon-phonon interaction channels can be derived from these trajectories which appear stochastic. These inter-dependent methods are adopted to successfully uncover the strong anharmonic phenomena while the traditional harmonic models fail dramatically, e.g., the negative thermal expansion of cuprite and the high temperature thermal stability of rutile.