Vibrational frequency without spin contamination error - Approximately spin projected force constant -.

An approximate spin projection (AP) method is applied to a second derivative of energy (Hessian) of the broken-symmetry (BS) method in order to eliminate a spin contamination error. Stretching frequencies of a singlet molecular oxygen (a1Δg) and a singlet carbene (1A1) are calculated by using the spin projected Hessian (AP Hessian). From those results, we found a significant spin contamination error in calculated frequencies of the singlet CH2 molecule. In addition, the AP method was also effective in calculating of a singlet-triplet energy gap of 1O2. [ABSTRACT FROM AUTHOR]