The effect of the exact exchange on the spin-state energetics of transition metal complexes is revisited
with an attempt to clarify its origin and with regard to performance of DFT methods. Typically, by
increasing an amount of the exact exchange in an exchange–correlation functional, higher spin states
are strongly stabilized with respect to lower spin states. But this is not always the case, as revealed from
the presented studies of heme and non-heme complexes, and of metal cations surrounded by point
charges. It is argued that the sensitivity of the DFT spin-state energetics to the exact exchange admixture
is rooted in the DFT description of the metal–ligand bonding rather than of the metal-centered exchange
interactions. In the typical case, where transition from a lower spin state to a higher spin state involves an
electron promotion from a nonbonding to an antibonding orbital, the lower spin state has a more
delocalized charge distribution and contains a larger amount of nondynamical correlation energy than
the higher spin state. However, DFT methods have problems with describing these two effects accurately.
This interpretation allows us to explain why the exact exchange admixture has a much smaller effect on
the energetics of spin transitions that involve only nonbonding d orbitals.