Perturbation scheme for determining the electronic excitation energy of molecules with open shells

The self-consistent perturbation method for determining energies of the gro und and excited states having the same symmetry is developed for molecules with open shells. Based on the formalism of the unrestricted Hartree-Fock m ethod with the restrictions imposed, the Hartree-Fock equations are obtaine d for the excited states. They are compatible with the orthogonality condit ions and satisfy the Brillouin theorem. The single-support perturbation the ory of the excited states is constructed, which possesses the same computin g advantages as the known Moller-Plesset perturbation theory. The possibili ties of the approach suggested are discussed by the example of calculations of the excitation energy of CH2 and BeF molecules.