The Unrestricted Hartree-Fock Method

The unrestricted Hartree-Fock method is a convenient method of calculating neutron form factors and hyperfine interactions, but because the method gives wave functions which are not eigenfunctions of S2, the validity of it must be questioned. It is shown that the unrestricted Hartree-Fock method gives spin densities which are approximately correct to first order provided certain exchange energies are small relative to `promotion' energies. Furthermore, it is shown that the wave functions obtained must be used directly and that it is a poor approximation to project out the unwanted parts after the energy minimization procedure has been performed.