Geometry optimization method based on approximate spin projection and its application to F2, CH2, CH2OO, and active site of urease

A new geometry optimization method based on an approximate spin projection (AP) procedure is proposed to eliminate a spin contamination effect in an optimized structure on a low spin (LS) state of a broken symmetry (BS) method. First, an energy gradient with the AP (AP gradient) is derived and it is applied to the geometry optimization of F2, CH2, and CH2OO in order to obtain their structures without the spin contamination. The optimization method corrects HCH angle of the CH2 about 10° in comparison with the BS method, and indicates that the spin contamination in optimized geometry of the BS LS states is considerably large. Next, an active site of Urease that consists of two Ni(II) ions is optimized by the AP method. The result indicates that structure of the active site is sensitive to a water molecule called W3. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007