We discussed the most stable coordination of O2 binding in the Cu2O2 core of type 3 copper proteins from a viewpoint of broken-symmetry (BS) DFT approach. This study put its focus on the active site of both oxy and deoxy form of hemocyanin as an example. We examined four conventional hybrid GGA functional sets that contain different proportion of exact exchange in a range of 20–50% within a BS way. To evaluate their net accuracy, the spin contamination errors caused by the triplet (S = 1) state on energy and energy gradient in the BS (S = 0) state were replaced by our approximate spin projection (AP) procedure. We also examined OO stretch vibrational frequencies and Gibbs free energies of O2 binding with the conductor-like polarizable continuum model. From the results, UBHandHLYP outperform the other functional sets, indicating that the Cu2O2 core in oxyHc has significant diradical character under AP-BS approximation. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009
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