Anharmonic phonons via quantum thermal bath simulations

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 behavior. 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. An accurate method, combining the quantum correlator approach with path integral molecular dynamics, was recently proposed to capture these effects [Morresi et al., J. Chem. Phys. 154(22), 224108 (2021)]. Unfortunately, this scheme may incur a substantial computational cost due to the increased number of degrees of freedom introduced by path integrals. In this work, we present an alternative that promises to mitigate this problem by accounting for NQEs 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.