Comparison of Hartree-Fock and Kohn-Sham determinants as wave functions

Kohn-Sham (KS) and Hartree-Fock (HF) determinants were used as the true man y-body wave functions for calculations of molecular energies, vibrational f requencies, and excited electronic states. The results justified common pra ctice, encountered in the sum over states theories, in which these two dete rminants are used as the first-order approximation of the wave function. Ho wever, a distinct behavior with respect to the second-order perturbation ca lculation was observed for the two cases. The Raleigh-Schrodinger perturbat ion theory, which is formally identical to the Levy-Gorling formalism and a nalogous to the usual HF/Moller-Plesset approach, leads to rather discourag ing results for the KS determinant. On the other hand, the rigid KS orbital s are more suitable for modeling of excited electronic states, which was in dicated by the obtained transition energies for model molecules. (C) 2000 J ohn Wiley & Sons, Inc.