The large set of experimental data allows us to contrast the performance of the approximations AM-1, ... AM-4, and we can assess their potentials in predicting (relative) lifetimes. AM-1 is the simplest of all, which uses the barriers from shifted CASPT2 PES. It depends only on the parameter ∆EHL, and provides satisfactory results in most cases, that compare well with the experiment, verifying that the non-rigorous description behind the model describes the main effect of the substitution rather well. This is also supported by previous other works using the same approximation to similar systems. The success of this model is based on its simplicity. The singlet and quintet potentials, as we saw in Figure , do not show much variations besides the vertical (energy) shifts upon substitution. Consequently, as long as the ∆EHLvalues are calculated with a reasonable accuracy, the results are fairly reliable. However, the limitations of this approach are also clear, as demonstrated in the current set when we are far from the reference. In this case it underestimates the substituent effects at the most negative resonance parameters (i.e., for large ED substituents). AM-2 is an extension of the previous approach that refines the model by actually calculating the barrier at the crossing of the two potentials. One could expect a confirmation of the previous approach, with improved results. However, we are faced with a larger scatter, and less reliability. The difference between these two approaches can be appreciated graphically in Figure . The loss in consistency is probably because of the DFT errors in the individual calculations: the final results depend on an extra complicated MECP optimization in addition to the ∆EHL. All of these calculations are challenging the accuracy limits of the DFT, and the errors are likely to add up unfavourably. The two other methods are based on a different approach, moving to a less classical, but still approximative quantum mechanical description. Among them, AM-3 is the one which is the more laborous and less approximative in principle, where frequencies are calculated for each complexes and states, and are used in the calculations of the Franck-Condon factors (Eq. 5). This approach often gives satisfactory results, but it seems very sensitive to the calculated vibrational frequencies and can fail when they differ from the others, as in the cases of 5 and 11, which is clear from the Table . However, decreasing the number of free parameters again results in a better performance. This is done by using a simplified version of this approach that has been introduced and amply used by A. Hauser to describe the quintet lifetimes for many spin-crossover systems, . It is based on the shifting of the same PES by the quintet-singlet energy differences, and using the same potentials and same base frequency. This approximation brings back the consistency, and provides results that are in excellent agreement with the experimental values (see Table and Figure ). Finally, we used the 12 system, which is likely the most sensitive to H-bonding to water among these complexes, to make a test on the effect of the solvent beyond a simple solvation model. Two water molecule were included in the calculations H-bonded to the OH group of the ligands, and AM-1, AM-3 and AM-4 were performed on it (after geometry optimization). (The system is denoted as OH • H 2 O and *12) in Tables and.) The results are closer to the experimental values, but the change is not dramatic. Therefore, this hints that the calculations without explicit solvent remain satisfactory for many of the systems less sensitive to H bonding.