A spin correction procedure for unrestricted Hartree-Fock and Møller-Plesset wavefunctions for singlet diradicals and polyradicals

A spin correction procedure for removing the spin contamination from unrestricted Hartree-Fock and Møller-Plesset wavefunctions for singlet diradicals is examined. Removal of the spin contamination is essential for calculation of reasonable singlet-triplet energy gaps. The convergence of the perturbation series is also substantially improved for polyradicals (spin contamination from more than the next-highest state) such as H2O, NH2 and CH2 with XH bonds twice their equilibrium lengths.