Most of the relevant characteristics of the potential landscape (beyond "vertical" optical transitions), such us the ∆EHL energy gap and the quintet-singlet energy barrier are very difficult to determine experimentally for strongfield complexes. However, a parameter that is closely related to the spin state transition and easily accessible by experiment is the lifetime of the quintet state. Therefore we set out to obtain an estimate of this parameter from a relatively simple theoretical procedure, one that can be applied for the purpose of ligand engineering, where computations need to be carried out for a large number of compounds in order to compare and evaluate them. For the parent [Fe(terpy)2] 2+ complex we previously calculated the potential energy surfaces of the singlet ground state and the quintet state at the CASPT2 level. These very accurate PES were computed along a nuclear coordinate that combines the two most relevant dimensions: the bond length between the Fe and the N of the central ring of terpyridine, r(Fe-Nax), and the angle defined by the 3 nitrogens of the terpyridine ligand φ(NNN). Having such a highly accurate PES available affords a simple and inexpensive way to estimate the lifetimes of the substituted derivatives from quantum chemical calculations. We used DFT to calculate the energy difference between the minima of the singlet and quintet states (∆EHL) for the other compounds, and simply shifted the CASPT2 PES of the parent complex to these values and read off the barrier heights (∆E ‡ i ) from the resulting graph. We found that both for [Fe(terpy)2] 2+ and for every substituted complex the height of the energy barrier between these states is inversely related to the ∆EHL. Subsequently we used the Arrhenius equation to determine the relative lifetimes for the room temperature decay of the quintet states of the substituted derivatives compared to that of the parent compound. The relative lifetimes with respect to