Molecular excitation energies computed with Kohn-Sham orbitals and exact exchange potentials

Exact local exchange potentials are computed for the diatomic molecules N-2, O-2, and CO, based on expansions in terms of molecular orbitals. Kohn-Sham orbitals and orbital energies are obtained for the exact exchange potentials, with correlation effects neglected. The ionization potential is in all cases found to be accurately predicted by the orbital energy of the highest occupied orbital. Limited configuration interaction calculations are performed based on the Kohn-Sham orbitals, and are found to yield accurate excitation energies for a series of singly excited states, in particular for N-2 and CO. Clearly inferior results are obtained from similar calculations by use of Hartree-Fock orbitals. Thus Kohn-Sham orbitals obtained with exact exchange potentials tend to have an interesting potential as basis for sophisticated many-body methods. (C) 2001 American Institute of Physics.