Low-Lying Electromeric States in Chloro-Ligated Iron(IV)-Oxo Porphyrin as a Model for Compound I, Studied with Second-Order Perturbation Theory Based on Density Matrix Renormalization Group

Employing second-order perturbation theory based on the density matrix renormalization group (DMRG-CASPT2), this work aims at providing a quantitative description of the spin state energetics of a chloro-ligated iron(IV)-oxo porphyrin as a model for the cytochromes P450 active species, also known as compound I (Cpd I). We explored DMRG-CASPT2 to its full extent with an extensive active space (up to 31 active orbitals) as well as a large number of renormalized states m (up to 10000). Different flavors of DMRG-CASPT2, using either the costly exact 4-particle reduced density matrix (4-RDM) or the cheaper cumulant approximated 4-RDM (cu(4)), were analyzed. All flavors essentially converge to similar relative energies between different spin states. Including a correction for the protein environment, we found a quartet Fe IV O ground state and, more importantly, a thermally accessible doublet Fe V O excited state that might directly contribute to the reactivity of this iron-oxo species. Our results also showed that cheaper approaches, such as CASPT2 based on a smaller active space or the cumulant approximation DMRG-cu(4)-CASPT2, are capable of accurately describing the spin state energetics of this species.