Correcting Systematic Errors in DFT Spin-Splitting Energetics for Transition Metal Complexes

Spin-splittings of 57 octahedral first-row transition metal complexes calculated with B3LYP are compared with a database of experimental spectra collected from the literature. A variety of transition metal centers in various oxidation states and multiplicities along with a number of different coordinating ligands are considered. Environmental effects have been included to enable reasonable quantitative comparison with experiment. The manifold of states is studied using initial guesses constructed from ligand field theory. A localized orbital correction (LOC) model, referred to as DBLOC-DFT (d-block localized orbital corrected density functional theory), systematically corrects B3LYP calculations using five parameters. The final results are a considerable improvement over conventional DFT, bringing the mean unsigned error (MUE) from 10.14 kcal/mol with a standard deviation of 4.56 to 1.98 kcal/mol with a standard deviation of 1.62. Depending on the relative multiplicities of the ground and excited states, it is shown that B3LYP*, which has 15% exact nonlocal exchange, can lead to larger errors with respect to experiment than B3LYP. Application to 7 complexes from Swart et al. [J. Phys. Chem. A 2004, 108, 5479.] and 14 small-gap spin-crossover complexes, from the literature, shows the DBLOC model provides good agreement with a variety of experimental data.