What do the Kohn-Sham orbitals and eigenvalues mean?

Kohn-Sham orbitals and eigenvalues are calculated with gradient-corrected f unctionals for a set of small molecules (H2O, N-2, CrH66-, and PdCl42-), va rying basis sets and functionals. The calculated Kohn-Sham (KS) orbital sha pes, symmetries, and the order and absolute energy of the associated eigenv alues are investigated and compared with those of Hartree-Fock (I-IF) and o ne-electron extended Huckel (eH) calculations, as well as experimental ioni zation potentials. The shape and symmetry properties of the KS orbitals are very similar to those calculated by HF and eH methods. The energy order of the occupied orbitals is in most cases in agreement among the various meth ods. The order of empty orbitals of a minimal basis set is sometimes interc hanged, within that group or with some orbitals resulting from a larger bas is calculation. Overall the KS orbitals are a good basis-as Baerends sugges ted-for qualitative interpretation of molecular orbitals. For the Kohn-Sham eigenvalues we find an approximately linear dependency of \epsilon(i)(KS) - epsilon(i)(HF)\ VS epsilon(i)(HF) (approximate to-IP) for the occupied as well as for the unoccupied orbital eigenvalues. We suggest an ax + b scali ng for quantitative interpretation of KS eigenvalues, at least if these are calculated utilizing commonly used functionals.