Finally, we have also performed ab initio calculations of the phonon lifetimes and linewidths and demonstrated that, provided an appropriate integration of reciprocal space is employed, the choice of phonon linewidth model is not crucial. Indeed, due to the likely unphysical narrowing of ab initio phonon linewidths at very low temperature, and the large computational burden of obtaining them in the first place, we do not suggest using temperature-dependent ab initio linewidths. While the NVT approximation appears to match the results obtained using ab initio linewidths quite well, their huge energy and temperature dependence (Figure ) can lead to numerical difficulties, and hence we do not recommend this model either. Indeed, given the similar results obtained from calculations with simple fixed linewidths, we suggest that a simple fixed-linewidth on the order of Γ = 0.1 -10 cm -1 is the most transferrable and economic method for molecular spin dynamics calculations. These conclusions, in addition to the open-source tools we have developed and used herein, open the door to quantitativelyaccurate and efficient spin-phonon calculations on molecular solids.