Ab Initio Calculations for Spin-Gaps of Non-Heme Iron Complexes.

We employed our recently proposed multireference approach CASPT2/CC to calculate the quintet-triplet gaps Δ of a series of non-heme Fe═O species and subsequently used these results to benchmark density functional theory (DFT) as well as two variants of local coupled-cluster approaches (DLPNO-CCSD(T) and LUCCSD(T0)). We showed that current implementations of the local coupled-cluster method are not sufficiently accurate. DLPNO-CCSD(T) systematically overstabilizes the quintet state, whereas LUCCSD(T0) overestimates the triplet one. This sort of systematic bias may be helpful in improving local correlation methods and can also be used as the basis for a simple correction scheme.