Partially restricted Hartree-Fock method for singlet excited states

A version of the partially spin-restricted Hartree-Fock method is proposed to construct a reference Slater determinant in calculations of singlet excited states having the same symmetry as the ground one. On the one hand, this approach can be used for a rather simple, in comparison with the fully unrestricted determinant, construction of a pure spin state. On the other hand, it is suitable, in contrast to the Roothaan theory for open shells, for the development of the perturbation scheme for taking into account correlation effects in an excited state, which retains the calculation advantages of the Moller-Plesset perturbation theory with single annihilation for the ground state. The efficiency of the method is demonstrated by calculations of electronic excitation energies for BH and CH⁺ molecules.