First-principles phonon lifetimes validated via inelastic neutron scattering

Phonon linewidths are a key component of quasiparticle theories of transport, yet ab-initio linewidths are rarely directly compared to inelastic neutron scattering (INS) results. Existing comparisons show discrepancies even at temperatures where perturbation theory is expected to work. In this work, we demonstrate that the reciprocal space voxel (q-voxel), which is the minimum region in reciprocal space sampled by INS, must be explicitly accounted for within theory in order to have a meaningful comparison with INS. We illustrate this in two ionic insulators having the fluorite structure: CaF₂ and ThO₂.  The quadratic and cubic phonon interactions are computed from DFT, and leading order perturbation theory is used to construct the scattering function at q-points. When the first-principles scattering function is computed within the same q-voxel as INS, the resulting linewidths are in reasonable agreement. Passing this test implies high fidelity of the phonon interactions and the approximations used to compute the Green’s function, serving as a critical benchmark of theory, and implying that other material properties should be reasonably predicted.