To obtain the lifetimes 𝜏 ?@ and linewidths Γ ?@ from first principles, we calculated third-order (anharmonic) force constants and modelled the phonon-phonon scattering processes explicitly for a grid of wavevectors in reciprocal space at a series of temperatures. Here we have used 2×2×2, 3×3×3, 4×4×4 and 5×5×5 q-point grids with 6, 8, 21 and 27 unique q-points, respectively, and performed the calculations at 10, 20, 30, 40, 50, 60 and 300 K. The ab initio linewidths vary both as a function of wavevector and mode energy (Figure ), but we find that their behaviour is relatively insensitive to the choice of grid (Figure ). There is also a marked temperature dependence (Figures and), arising from larger scattering probabilities as the phonons are more heavily populated at elevated temperature, which above 30 K is well approximated as Γ ∝ 𝑇, in agreement with the high-temperature limit of Equation . However, the linewidths predicted by Equation 1 differ substantially from our ab initiocalculated values, especially at low and high phonon energies (Figure ). The ab initio phonon linewidths for 1 at 300 K are on the order of 0.1 -40 cm -1 (corresponding to lifetimes on the order of 100 -0.1 ps, respectively), and some modes become much longer lived at low temperatures with lifetimes up to 4.5 ns (linewidths of 0.001 cm -1 ) at 10 K. However, even the longest of these ab initio lifetimes is still orders of magnitude shorter than the experimental spin lifetimes for this compound, which are seconds to hundreds of microseconds, justifying the commonly assumed Born-Markov approximation for molecular spin dynamics. To the best of our knowledge the present study represents one of very few explicit calculations of the phonon linewidths and lifetimes for a molecular crystal, and the only such calculation for a molecular magnet.