Theoretical studies of the effect of orientation of ligands and spin contamination error on the chemical bonding in the FeO2 core in oxymyoglobin

We performed broken-symmetry (BS) B3LYP calculations on several model complexes of the active site of oxymyoglobin. The present study focuses on the effects of the orientations of His64 and His93 on the chemical bonding of the FeO2 core in the ground (S = 0) state. The orientation of binding O2 molecule as well as axial His93 drastically changes during the optimization when using the BS method. In order to make this point clear, the spin contamination error involved in the BS singlet state is removed from both the energy and its gradient by an approximate spin-projection (AP) method. In most cases, the attempt to correct the geometry results in failure, indicating the Heisenberg model cannot be always applicable to this system. Judging from the difference in the structural parameters between the BS and corrected BS (AP) structure, the spin contamination error on the geometry of the FeO2 core is small. Based on these results, we suggest that the geometry optimization using a small model complex such as FeO2(Im)(Por) (Im, imidazole; Por, porphyrin) provides a local minima, in which π bond of Fe-O2 core is artificially stabilized by rotating the O2 molecule.