Abstract
Ten low-lying electronic states of Fe(II) porphine, ⁵A_1g, ⁵E_g, ⁵B_2g, ³A_2g, ³B_2g, ³E_g(A), ³E_g(B), ¹A_1g, ¹B_2g and ¹E_g states, are studied with multireference Møller–Plesset perturbation theory (MRMP) with complete active space self-consistent field (CASSCF) reference functions. Triplet and singlet states are significantly multiconfigurational in character. Ten low-lying states are computed to be within a 2 eV span and the ⁵A_1g state is predicted to be the lowest. At the CASSCF level, all quintet states are lower in energy than triplets. This tendency is reversed except the ⁵A_1g state after perturbation theory is applied. Among controversial candidates for the triplet ground state, the ³E_g state is computed to be more stable by 0.18–0.23 eV than the ³A_2g and ³B_2g states.