Perturbation Scheme for Determining the Electronic Excitation Energy of Molecules with Open Shells

摘要

The self-consistent perturbation method for determining energies of the ground and excited states having the same symmetry is developed for molecules with open shells. Based on the formalism of the unrestricted Hartree-Fock method with the restrictions imposed, the Hartree-Fock equations are obtained for the excited states. They are compatible with the orthogonality conditions and satisfy the Brillouin theorem. The single-support perturbation theory of the excited states is constructed, which possesses the same computing advantages as the known Moller-Plesset perturbation theory. The possibilities of the approach suggested are discussed by the example of calculations of the excitation energy of CH_2 and BeF molecules.