To maximize correspondence, we only compare Fe and Ru complexes with identical ligands (i.e., they only differ by metal identity) that have successful DFT calculations for all studied intermediates. This produces a final set of 222 Fe and Ru catalysts. Reaction energetics are compared across identical spin states (Supporting Information Table ). For spin splitting energy comparisons, we restrict ourselves only to the spin splitting energy between the IS and LS states (i.e., DEI-L = E(IS) -E(LS)) for a given intermediate because of the high failure rate of DFT calculations of Ru complexes in the HS state, particularly for the complexes with macrocyclic ligands (Supporting Information Table ).