Excitation Energies from a Partially Spin-Restricted Wave Function

A singe Slater determinant consisting of restricted and unrestricted, in spins, parts is proposed to construct a reference configuration for singlet excited states having the same symmetry as the ground one. A partially restricted Hartree-Fock approach is developed to derive amended equations determining the spatial molecular orbitals for singlet excited states. They present the natural base to describe the electron correlation in excited states using the well-established spin-annihilated perturbation theories. The efficiency of the proposed method is demonstrated by calculations of electronic excitation energies for the Be atom and LiH molecule.