To probe the origin of the magnetic reversal mechanisms in 1-Dy further, we performed ab initio spin dynamics using our recently developed methods , which involves: (i) optimisation of the crystal structure and calculation of phonon modes with periodic DFT; (ii) calculation of embedded molecular electronic structure and spin-phonon coupling with CASSCF-SO; and, (iii) the use of a semi-classical master equation to simulate one-and two-phonon molecular spin dynamics and magnetic relaxation. The optimised crystal structure is a reasonable match with the experimental structure, but with a slightly reduced N-Dy-N angle of 146° compared to the XRD geometry. The calculated phonon dispersion (Figure ) shows several small off-Γ imaginary modes (< 20i cm -1 ) that were not computationally feasible to remove due to the large size of the primitive unit cell, however their presence does not impact the results (Figure ). The calculated phonon density of states (DoS; Figure ) is dense at low energies and sparse at higher energies, as expected for a molecular crystal. Calculation of the spin-phonon coupling and magnetic reversal rates (see Supporting Information for details) gives excellent agreement with experiment (Figure ). (blue) with 1-Dy (black). b, Ab initio calculation of the magnetic relaxation rates for 1-Dy. Purple points are the experimental relaxation rates for 1-Dy (Table ). Solid black line gives the ab initio calculated Orbach relaxation rate and dashed black line gives the ab initio calculated Raman relaxation rate.