including the infinite crystalline electrostatic potential, and considering both single-phonon and two-phonon transitions, with a fixed Γ = 0.1 cm -1 (red) and ab initio mode-and temperaturedependent linewidths (pink points and dashed lines). The error bars on the experimental data points denote one estimated standard deviation of the distribution of relaxation rates. We now examine the impact of using the ab initio phonon linewidths. In the Orbach region, the rates obtained using mode-and temperature-dependent linewidths coincide with the fixed linewidth calculations using Γ = 1 cm -1 (Figure ). For the Raman region, the rates with mode-and temperature-dependent linewidths are close to those with fixed Γ = 10 cm -1 at 60 K, but increasingly approach the smaller fixed-linewidth calculations at lower temperature, crossing the Γ = 0.1 cm -1 rates between 20 and 10 K (Figure ). As the Raman rates are more strongly affected by the choice of linewidth than the Orbach rates, it is unsurprising that the extreme narrowing of some of the ab initio phonon linewidths at low temperatures has a marked impact on the relaxation rates. However, we note that the profile of the relaxation rates calculated using ab initio mode-and temperature-dependent linewidths does not agree with the experimental data at the lowest temperatures, and in particular level off at low temperature while the experimental rates continue to decrease. We therefore suggest that the extreme narrowing of the ab initio linewidths at low temperatures is overestimated, and that there are likely other sources of phonon scattering (such as boundary effects, impurities, defects and/or disorder) in real crystals that would lead to shorter phonon lifetimes, and therefore broader linewidths, at lower temperature than estimated by our DFT calculations on a perfect infinite crystal.