Theoretical studies on the structure and effective exchange integral (Jab) of an active site in oxyhemocyanin (oxyHc) by using approximately spin-projected geometry optimization (AP-opt) method

The effect of a spin contamination error in an optimized geometry of Cu2O2 in the active site of oxyHc is examined by using broken-symmetry (BS) and AP-opt methods with hybrid density functional methods. The error was 3–5° on the Cu–O–Cu angle and 0.07–0.13 Å on the reaction coordinate R. The error in the optimized structure strongly affects the calculated Jab values such as 196 and 553 cm−1 for UBHandHLYP and UB3LYP methods, respectively. The underestimated Jab value of UB3LYP becomes close to the experimental one by the correction of the structure with the AP-opt method.