On the Role of Valence and Semi-Core Electron Correlation on Spin-Gaps in Fe(II)-Porphyrins

The role of valence and semi-core correlation in differentially stabilizing the intermediate spin-state of Fe(II)-porphyrins is analyzed. CASSCF treatment of the valence correlation, with a (32,34) active space containing metal 3d, 4d orbitals
and the entire π system of the porphyrin, is necessary to stabilize the intermediate spin-state for this system. Semi-core correlation provides a quantitatively significant (~1.5 kcal/mol) but less important correction. Accounting for both types of correlation enlarges the (3Eg5A1g) spin-gap to −5kcal/mol.