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 th ey nevertheless form a single-particle system, which may serve as a ba sis for perturbation theory or variational methods. Rayleigh-Schroding er perturbation series for some molecular test cases, taking different Kohn-Sham quasi-particle systems as zeroth order, are examined in com parision to Moller-Plesset perturbation theory. Particular emphasis is given to the convergence properties of the series. The latter is most important regarding recent Ansatze for improving modem density functi onals by perturbative methods. We find that these Kohn-Sham based pert urbation series are in general divergent. Also, Kohn-Sham orbital base d multi-reference CI calculations are performed with emphasis on excit ed states as Hartree-Fock orbitals generally provide a poor descriptio n of excited states.