  We present a first-principles investigation of spin-phonon relaxation in a
molecular crystal of Co(II) single-ion magnets. Our study combines electronic
structure calculations with machine-learning force fields and unravels the
nature of both the Orbach and the Raman relaxation channels in terms of
atomistic processes. We find that although both mechanisms are mediated by the
excited spin states, the low temperature spin dynamics is dominated by phonons
in the THz energy range, which partially suppress the benefit of having a large
magnetic anisotropy. This study also determines the importance of
intra-molecular motions for both the relaxation mechanisms and paves the way to
the rational design of a new generation of single-ion magnets with tailored
spin-phonon coupling.
