A more rigorous but still straightforward way of estimating the barrier heights is to determine the minimum energy crossing point (MECP) between the potential energy surfaces of the singlet and quintet states by DFT (B3LYP*/TZVP), see Figure This is achieved by performing a geometry optimization minimizing the energy difference between these states. In this case the barrier is simply the energy difference between the MECP and the HS minimum. The MECP-geometries are located in between those of the HS and the LS along the relevant modes, closer to the former, and the contributing coordinates are again the r(Fe-Nax) bond lengths and the φ(NNN) angles of the ligands (Supporting Information Table ). The quintet-tosinglet energy barriers calculated from the MECP energies show the same inverse correlation with ∆EHL as we found before. The Arrhenius equation is used to predict the lifetime of the quintet state from these barrier heights as well. This approach will be referred to as AM-2.