CCL:Orbitals

[Mauricio L Cafiero <mcafiero:at:U.Arizona.EDU>]

hello:
      If we take quantum mechanics to be valid, and we accept that the
 two particle coulomb interaction is the predominant potential energy term
 in the hamitonian, then MO's are physically meaningless. They are simply
 artifacts of approximations, a zero-order wavefunction. QM could have
 evolved completely without orbitals (except for H-atom of course). I think
 the reason we jumped on
 orbitals so early in history is that they are a fantastic approximation
 and that orthogonal orbitals save a lot of time computationally.
 It is the very excellence of the orbital approximation that makes
 orbitals out to be more than they are. As was said before, they are
 conceptually easy to grasp. As far a Koopmans', it makes no explicilt
 reference to orbitals. It is simply that the orbital energy is what
 remains when you subtract the energies of the two wave functions
 in the single determinant approximation. I also have to say that PES
 does not always agree well with orbital diagrams, at least not
 quantitatively.
 Mauricio Cafiero
 Postdoctoral Research Associate
 Computational Chemistry Team
 National Institute of Standards and Technology
 Gaithersburg, Maryland