cm -1 at 300 K from Equation , but is ca. 1 -10 cm -1 as calculated ab initio, Figures and) such that the numerical integration over the phonon lineshapes when computing the Raman mechanism becomes problematic, and we therefore report these results for T ≤ 46 K. We have also previously shown that the NVT linewidths of the high-energy phonons drastically narrow at low temperature, resulting in unphysical Orbach rates, which again we find here when using anti-Lorentzian lineshapes. Nevertheless, we find that in both the Orbach and Raman regions the NVT expression gives rates that are close to those obtained using a fixed Γ = 10 cm -1 (Figures and), and, in fact, gives rates nearly identical to those obtained by fixing the linewidths to their values calculated ab initio at 300 K, which themselves agree well with the rates obtained using the mode-and temperature-dependent linewidths above ca. 30 K (Figures and). Overall, however, we obtain the best agreement with the experimental rates by using a simple fixed linewidth of Γ = 0.1 cm -1 (Figure ), but really there is little difference when using Γ = 1 cm -1 when the largest 5×5×5 q-point grid is employed (Figure ).