To examine the influence of ab initio phonon linewidths on the spin dynamics, we proceeded to calculate the spin-phonon coupling and magnetic relaxation rates (see Methods). We first examined the spin dynamics under the gas-phase ansatz by optimising the geometry and obtaining the vibrational modes of an isolated molecule of 1 (see Methods). The calculated single-phonon Orbach magnetic relaxation rates, using the gas-phase vibrational modes and fixed linewidths, show excellent agreement with the experimental measurements at hightemperature (Figure ). As we have shown previously, the absolute rates show a significant dependence on the choice of linewidth, with broader linewidths leading to faster relaxation rates. This behaviour occurs for single-phonon processes because there is only one possible phonon energy ℏ𝜔 = K𝐸 M -𝐸 N K (where 𝐸 M and 𝐸 N are the final and initial electronic state energies) that can cause a spin transition, and a larger linewidth gives a larger probability of overlap between K𝐸 M -𝐸 N K and the calculated vibrational energies. We find that the best agreement is obtained with Γ = 1 cm -1 (Figure ), which is consistent with the approximate centre of the distribution of ab initio phonon linewidths at 50 K (Figure ).