orbitals

   Leif Laaksonen makes an important point: only observables
 are "real" in QM.  So the question arises: is it possible to
 define orbitals as QM observables.  It entirely depends on
 how you define the word "orbital".  If one says orbitals are
 one-electron functions and nothing more, then of course orbitals
 can be made to be QM observables.  For example, just integrate
 an exact squared wavefunction over all electron coordinates
 but one to get the electron density, then take the square
 root, multiply by a phase factor and you get "density orbitals".
 [G Hunter, Int J Quantum Chem 9 (1975) 237].   They may not be
 very useful as orbitals, but they are "real" in the sense of
 QM observables.
     Orbitals are always defined as one-electron functions,
 but THAT does not make them "unreal".  The electron density
 is a one electron function and very real for any N-electron
 system, not only for the H atom.
     One "ab initio" way to define orbitals is to start from
 exact wavefunctions and calculate the overlap of a N-electron
 wavefunction (neutral) and the ground and various excited
 states a (N-1)-electron wavefunction (ion) (Dyson orbitals are
 defined in such a way I think).  I'm sure there are definitions
 that will make such orbitals unique, but does that make them
 QM observables?  I don't think so, but I'd like to hear from
 the CCL community on that.  There are other ways to define orbitals
 > from exact wavefunctions, I'm sure: are any of them QM observables?
     As for Kohn-Sham orbitals, they may not be real individually
 but the N lowest energy ones squared and summed give the true
 electron density (an observable) and the negative of the KS HOMO
 energy is exactly equal to the first IP (another observable) so
 there are at least some bits of reality built in KS orbitals!
 Here's some relevant articles:
 1) Kohn, Becke, Parr, J Phys Chem 100 (1996) 12974
 2) "Kohn-Sham Orbitals and Orbital Energies: Fictitious Constructs but
    Good Approximations All the Same"
    by S. Hamel, P. Duffy, M.E. Casida, and D.R. Salahub
    Journal of Electron Spectroscopy and Related Phenomena (2002?)
 3) Chong, Gritsenko, Baerends, J Chem Phys 116 (2002) 1760
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