Structure and Spin-State Energetics of an Iron Porphyrin Model: An Assessment of Theoretical Methods

The ability of unrestricted Hartree–Fock (UHF), Møller–Plesset (MP2), density functional theory (DFT), and hybrid density functional/Hartree–Fock methodologies to describe the structure and spin-state energetics of iron porphyrins was assessed. In the first place, these techniques have been applied to Fe, Fe+, Fe2+, and Fe3+ for which HF calculations overestimate energy gaps, favoring stabilization of higher multiplicity states. DFT shows the opposite trend at the GGA level, with some improvement using the hybrid schemes B3LYP and half-and-half. We use the hybrid functionals to explore the dependence of the spin state with the iron displacement out of the porphyrin plane in the five-coordinate system, for which a high-spin ground state has been experimentally determined. The possibility of spin crossover, proposed in previous studies, is examined. Finally, the hybrid methodologies are applied to the computation of the oxyhemoglobin model. The B3LYP description of the electronic structure of both penta and hexa coordinate model systems is consistent with previous theoretical calculations and with experimental information of deoxy and oxy hemoglobin. © 2002 John Wiley & Sons, Inc. Int J Quantum Chem 87: 158–166, 2002