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RASPT2 study of the valence excited states of an iron–porphyrin–carbonyl model complex

Multireference wavefunction calculations of the singlet valence excited states of an iron–porphyrin–pyrazine–carbonyl complex up to the Soret band (about 3 eV) are presented. This complex is chosen to be a model for the active site of carboxyhemoglobin/carboxymyoglobin. The investigations are performed at the restricted active space second‐order perturbation (RASPT2) level involving an extended active space on the porphyrin ligand in addition to the active orbitals needed for the description of the metal–ligand interactions. Metal‐to‐ligand‐charge‐transfer states d → π* and some metal‐centered d → d transitions are found in the lowest part of the spectrum, below the first π → π* intraporphyrin transitions (Q band). Doubly excited states involving simultaneous intraporphyrin and metal‐centered excitations are found in the vicinity of the second set of intraporphyrin transitions (the so‐called Soret band). The effect of the extension of the active space on the porphyrin ligand beyond the Gouterman's orbitals set is investigated together with the effect of inclusion of the ionization potential electron affinity shift in the RASPT2 treatment. © 2019 Wiley Periodicals, Inc.