Can large active‐space CASSCF calculation make sense to the reaction analysis of iron complex? A benchmark study of methane oxidation reaction by FeO

A methane oxidation reaction by FeO+ cation was theoretically investigated based on the density functional theory (DFT) and the complete active‐space self‐consistent field (CASSCF) method as well as the coupled‐cluster singles, doubles, and perturbative triples (CCSD(T)) to explore the active‐space dependency to computational analyses in such strongly correlated reaction systems. A small active‐space CASSCF(5e in 5o) calculation, which only includes five 3d orbitals of the Fe atom in the active‐space, showed remarkable difference both in energy and geometry compared to those computed by the DFT and CCSD(T) methods. Interestingly, a large active‐space CASSCF(17e in 17o) calculation, which includes almost all the valence orbitals gives a qualitative agreement with either the DFT or the CCSD(T) results in the first half part of the reaction, although it varies from them in the latter half part. Therefore, it is indicated that the active‐space dependency is serious in some part of the reaction and the small active‐space CASSCF might lead a wrong discussion. We further investigated the optimized geometry of the intermediate complex with the small and the large active‐space CASSCF methods as well as the CCSD(T) method, and found that the CASSCF(5e in 5o)‐optimized geometry is considerably different from the others. In consequence, a small active‐space CASSCF/CASPT2 calculation does not really work for such a strongly correlated reaction system even qualitatively, and a sophisticated assessment using the large active‐space CASSCF/CASPT2 method will be indispensable. © 2018 Wiley Periodicals, Inc. Methane oxidation by FeO+ has been revisited to investigate whether the DFT and the standard CASSCF/CASPT2 approaches can really give reliable results in such a strongly correlated reaction system. The authors' assessment using large active‐space DMRG‐CASSCF and CCSD(T) methods clearly indicated that a small active‐space CASSCF/CASPT2 method doesn't improve the DFT results, and may even get worse both for energy and geometry. The final conclusion however, is still unclear due to computational limitations.