Kohn-Sham orbitals for many-body perturbation theory and CI

Although the physical significance of Kohn-Sham orbitals is only given by the ground state density of the Kohn-Sham quasi-particle system they neverthless form a single-particle system, which may serve as a basis for perturbation theory or variational methods. Rayleigh-Schrِdinger perturbation series for some molecular test cases, taking different Kohn-Sham quasi-particle systems as zeroth order, are examined in comparision to Mّller-Plesset perturbation theory. Particular emphasis is given to the convergence properties of the series. The latter is most important regarding recent Ansنtze for improving modern density functionals by perturbative methods. We find that these Kohn-Sham based perturbation series are in general divergent. Also, Kohn-Sham orbital based multi-reference CI calculations are performed with emphasis on excited states as Hartree-Fock orbitals generally provide a poor description of excited states.