Role of Valence and Semicore Electron Correlation on Spin Gaps in Fe(II)-Porphyrins

The role of valence and semicore correlation in differentially stabilizing the intermediate spin state of Fe(II)-porphyrins is analyzed. For CASSCF treatments of valence correlation, a (32,34) active space containing metal 3d, d' orbitals and the entire pi system of the porphyrin is necessary to stabilize the intermediate spin state. Semicore correlation provides a minor (-1.6 kcal/mol) but quantitatively significant correction. Accounting for valence, semicore, and correlation beyond the active space enlarges the (E-3(g) - (S)A(1g)) spin gap to -5.7 kcal/mol.