Chemical insights into the electronic structure of Fe(II) porphyrin using FCIQMC, DMRG, and generalized active spaces

Stochastic-CASSCF and DMRG procedures have been utilized to quantify the role of the electron correlation mechanisms that in a Fe-porphyrin model system are responsible for the differential stabilization of the triplet over the quintet state. Orbital entanglement diagrams and CI-coefficients of the wave function in a localised orbital basis allow for an effective interpretation of the role of charge-transfer configurations. A preliminary version of the Stochastic Generalized Active Space Self-Consistent Field method has been developed and is here introduced to further assess the pi-backdonation stabilizing effect.
By the new method excitations between metal and ligand orbitals can selectively be removed from the complete CI expansion. It is demonstrated that these excitations are key to the differential stabilization of the triplet, effectively leading to a quantitative measure of the correlation enhanced pi-backdonation.