Theoretical study of the electronic ground state of iron(II) porphine

Ten low-lying electronic states of Fe(II)-porphine, the 5A1g, 5Eg, 5B2g, 3A2g, 3B2g, 3Eg(A), 3Eg(B), 1A1g, 1B2g and 1Eg states, are studied with multireference Møller–Plesset perturbation theory with complete active space self-consistent field (CASSCF) reference functions. Triplet and singlet states are significantly multiconfigurational in character. The ten low-lying states are computed to be within a 2 eV span and the 5A1g state is predicted to be the lowest. At the CASSCF level, all the quintet states are lower in energy than the triplets. This tendency is reversed, except for the 5A1g state, after perturbation theory is applied. Among controversial candidates for the triplet ground state, the 3Eg state is computed to be more stable, by 0.18∼0.23 eV, than the 3A2g and 3B2g states.