First-principles calculations of phonon frequencies, lifetimes and spectral functions from weak to strong anharmonicity: the example of palladium hydrides

TL;DR

This paper presents a combined computational approach using the variational stochastic self-consistent harmonic approximation and density-functional perturbation theory to accurately calculate anharmonic phonon properties, validated on palladium hydrides.

Contribution

It introduces a novel method that efficiently computes anharmonic phonon linewidths and spectra without needing fourth-order coefficients, applicable to strongly anharmonic materials.

Findings

Efficient calculation of phonon linewidths and shifts in anharmonic crystals.

Inelastic neutron scattering spectra show complex structures due to strong anharmonicity.

Validation on palladium hydrides demonstrates the method's effectiveness.

Abstract

The variational stochastic self-consistent harmonic approximation is combined with the calculation of third-order anharmonic coefficients within density-functional perturbation theory and the "$2n+1$" theorem to calculate anharmonic properties of crystals. It is demonstrated that in the perturbative limit the combination of these two methods yields the perturbative phonon linewidth and frequency shift in a very efficient way, avoiding the explicit calculation of fourth-order anharmonic coefficients. Moreover, it also allows calculating phonon lifetimes and inelastic neutron scattering spectra in solids where the harmonic approximation breaks down and a non-perturbative approach is required to deal with anharmonicity. To validate our approach, we calculate the anharmonic phonon linewidth in the strongly anharmonic palladium hydrides. We show that due to the large anharmonicity of hydrogen optical modes the inelastic neutron scattering spectra are not characterized by a Lorentzian line-shape, but by a complex structure including satellite peaks.