Spin unrestricted calculations using density functional theory can yield wave functions with spin contamination. In conventional post Hartree-Fock calculations (such as Moller-Plesset perturbation theory), spin projection can ameliorate some of the problems caused by spin contamination. However, spin projection can seriously degrade the quality of potential energy surfaces calculated by density functional methods, just as spin projection can yield poor results for Hartree-Fock potential energy surfaces.