13 kcal/mol. Furthermore, studies where GGA exchange and meta-GGA exchange were tuned to reproduce spin splitting energies evaluated with CASPT2 showed that the sensitivity of spin state ordering to HF exchange strongly depends on ligand-field strength . Several studies of SCO behavior have been carried out with DFT functionals, and these disagreed on which functionals lead to accurate predictions. Some of these studies found B3LYP 106 or a modified variant (aHF = 0.15) to perform best in predicting spin-splitting energies of Fe(II) SCO complexes, while recommending the tuning of the exact exchange parameter to verify the validity of the functional against experiments or correlated wavefunction theory methods 108 . However, other studies working with different test sets comprising small Fe(II/III/IV) and Co(II) complexes, some of which show SCO behavior, found OPBE, TPSSh, M06-L, 114 or B2PLYP to give the best results. In addition, recent small-scale studies using DFT-based methods on four homoleptic octahedral Fe(II) SCO model complexes with small monodentate ligands (NCH, NH3, H2O, CO) suggested that evaluation of non-empirical functionals on self-interaction-corrected densities and methods such as locally scaled self-interaction correction 118 can mitigate density errors and improve accuracy of spin splitting energies. However, it is unclear if these observations can be extended to larger SCO complexes or SCO complexes with diverse ligands and ligand denticities. Given that prior studies are mostly small in scale and disagree about what functionals best predict SCO behavior, a significantly larger set of Fe(II) SCO complexes is necessary to determine which functionals make accurate SCO predictions.