Limits of Coupled-Cluster Calculations for Non-Heme Iron Complexes

In a large variety of studies, the coupled-cluster method with singles, doubles, and perturbative triples (CCSD(T)) is used as a reference for benchmarking the performance of density functional theory (DFT) functionals. In the case of open-shell species, this theory can be applied in different forms depending on the restricted or unrestricted treatment of spin. In this study, we show that these different approaches can produce results which deviate by ∼5 kcal/mol for different species on the potential energy surfaces. This was demonstrated for a simple model of the C-H activation carried out by non-heme iron enzymes. Assessing the limits of CCSD(T) prior to its use as a general benchmark tool is warranted. This was done using higher-order coupled-cluster calculations as well as multiconfigurational second-order perturbation theory (CASPT2), since iron-oxo species present some multireference character. Furthermore, we tested two different implementations of the local coupled-cluster method and compared them to the CCSD(T) results, showing that even though these novel approaches are promising, without further developments they appear not to be suitable for describing two-state reactivity of the system investigated in the current study. Additionally, we implemented and assessed the performance of the hotspot approach for the local unrestricted CCSD(T) scheme which aims at reducing the pair error for systems containing transition metals.