# CCL:Orbitals

*From*: Mauricio L Cafiero <mcafiero:at:U.Arizona.EDU>
*Subject*: CCL:Orbitals
*Date*: Wed, 28 May 2003 10:50:48 -0700 (MST)

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