Toward accurate spin-state energetics of transition metal complexes

The ability to conclusively predict relative energies of different spin states for transition metal complexes and metal sites in enzymes is highly relevant in (bio)inorganic chemistry, but poses an outstanding challenge for quantum chemical calculations. We discuss representative applications of wave function theory (WFT) and density functional theory (DFT) methods to compelling transition metal complexes and models of active sites in metalloenzymes, aiming not only to resolve some existing controversies with spin-state predictions, but also to develop reliable, yet efficient computational protocols for the problem of spin-state energetics. The presented examples confirm that DFT results are highly dependent on the choice of exchange-correlation functional and the optimal choice is not universal, even when considering different spin-state gaps in the same molecule. Mechanistic consequences of these issues are emphasized for spin-forbidden ligand binding. Among the WFT methods, a high accuracy of the single-reference coupled cluster CCSD(T) method is confirmed by a number of examples studied, including systems with noticeable nondynamic correlation effects. However, we point out that in some cases controversial results are obtained also from WFT calculations calling for further benchmarking of quantum chemistry methods with respect to quantitative experimental data of spin-state energetics. In this regard, the environmental (solvation or crystal packing) effects on relative spin-state energetics must be accounted for when comparing theory with experiment.