The performance of hybrid DFT for mechanisms involving transition metal complexes in enzymes

The accuracy of density functional theory with the B3LYP functional is reviewed for systems of relevance to transition-metal-containing enzymes. Calculated energies are commonly within 3-5 kcal/mol of the correct values; however, some exceptions have appeared in the literature and are discussed here. For example, the binding of NO and that of O(2) to metal centers have for some time been known to be underestimated. Most barriers for chemical reactions are overestimated except those involving hydrogen (or proton) transfer, which instead tend to be underestimated. A minor general improvement of the accuracy can probably be obtained by slightly reducing the amount of exact exchange in the B3LYP functional.