  Molecular vibrations are increasingly seen as a key factor for spin dynamics
in single-ion magnets and molecular spin qubits. Herein we show how an
inexpensive combination of molecular dynamics calculations and a crystal field
analysis can be employed to obtain a dynamical picture of the crystal field
splitting. We present the time evolution during 500ns of the spin energy levels
in a terbium complex. Our calculations evidence an amplitude of up to tens of
cm-1 for the oscillations of the spin energy levels in the fs time scale.
Crucially, we also see that the oscillations average out and practically
disappear at longer time scales, thus ruling out the modulation of spin energy
levels by phonons of long wavelength $\lambda$ (up to $\lambda$ = 40 $\AA$).
Our results are compatible with the common approximation of focusing on local
vibrations, but at the same time highlight the risks of assuming that the spin
energy levels are time- and temperature-independent.
