Spin-State Energetics of Fe Complexes from an Optimally Tuned Range-Separated Hybrid Functional

We assess the performance of the optimally tuned range-separated hybrid (OT-RSH) functional approach in predicting the ground-state electronic configuration and spin-state energetics of complexes that can potentially exhibit multiple spin configurations. To that end, we investigate eight iron complexes: four spin-crossover complexes, for which reference data from other approximate density functionals are available, and four smaller complexes, for which reference ab initio data are available. We show that the spin-state energetics are mostly governed by the percentage of short-range exact exchange and are only weakly influenced by the choice of the range-separation parameter. However, the electronic structure, especially the fundamental gap, is much more sensitive to the range-separation parameter. We further find that correct prediction of the ground state in spin-crossover compounds requires a reduction in the amount of short-range exact exchange, likely owing to a larger role of static correlation.