Anharmonic phonons via quantum thermal bath simulations

Abstract

Lattice vibrations within crystalline solids, or phonons, provide information on a variety of important material characteristics, from thermal qualities to optical properties and phase transition behaviour. When the material contains light ions, or is subjected to sufficiently low temperatures and/or high pressures, anharmonic and nuclear quantum effects (NQEs) may significantly alter its phonon characteristics. Unfortunately, accurate inclusion of these two effects within numerical simulations typically incurs a substantial computational cost. In this work, we present a novel approach which promises to mitigate this problem. The scheme leverages the recently introduced quantum correlators approach for the extraction of anharmonic phonon frequencies from molecular dynamics data. To account for NQEs without excessive increase of the computational cost, we include nuclear quantum effects via the quantum thermal bath (QTB) method. This is the first full exploration of the use of QTB for the calculation of phonon dispersion relations. We demonstrate the noteworthy efficiency and accuracy of the scheme, and analyze its upsides and drawbacks by first considering 1-dimensional systems, and then the physically interesting case of solid neon.