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<br>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 (<sup>3</sup>E<sub>g</sub>−<sup>5</sup>A<sub>1g</sub>) spin-gap to −5kcal/mol.<br>