Ground and excited states of carboxyheme: a SAC/SAC-CI study

The SAC (symmetry adapted cluster)/SAC-CI method is applied to calculations of the ground and excited states of carboxyheme (FeC24N6OH16). The excited states are calculated up to 7.8 eV. The calculated excitation energies and oscillator strengths reproduce well the electronic spectrum. The Q bands are explained by the excitations within Goutermanʹs 4 orbitals. The B, N, L and M bands are explained by the excitations within ‘6 orbitals’, in which the lower porphyrin π orbitals, 44a′ and 45a′, are added to the 4 orbitals. The B band shoulder in the higher energy side has a d, σ-d∗, σ∗, CO∗ nature. The states higher than the M band are characterized by the d-π∗ and π-d∗ states; therefore, the intensities are predicted to be small.