In the above approximation, which we also used in re- cent works, we applied a classical description that focuses on the fraction of molecules which can surpass the energy barrier. While this approach seems to hold for smaller differences, it is clear that the prediction of the lifetimes become less satisfactory as ∆EHL differs significantly from that of the reference compound 4. The description can be improved by adopting a more quantum mechanical viewpoint, where the transition processes from each populated quintet vibrational state are taken into account; these proceed to the corresponding singlet states, with which they have significant overlap. Thus below the barrier, this includes tunneling processes. A theoretical account based on this was given by E. Buhks et al. to describe the spin conversion processes in solution, which A. Hauser modified and successfully applied to obtain low-temperature transition (tunneling) rates in solids. Although it was exactly the [Fe(terpy)2] 2+ system that deviated from the otherwise very good overall agreement with the spin-crossover data, in certain matrices at low temperatures, probably due to the failure of the single configuration coordinate (SCC) model coupled to mentioned effects, in a room temperature solution the constrains posed by solid-state and lowtemperature (low-energy) effects can be lifted, and the SCC model can regain its validity; the relative success of the above discussed simple classical model already indicates that relaxation along a single mode can explain most of the variations. Nevertheless, it is interesting to test whether a simple quantum mechanical description, which is based on the Fermi golden rule expression of the rate constant, and involves the tunneling processes can give a better agreement with the experiment.