Re: CCL:Orbitals

E. Lewars:
 > Are MOs physically real? This is a meaningful question only if there
 > is some experiment or observation that could provide an answer _yes_
 > or _no_.  Is there, at least in principle, such an experiment or
 > observation?
 I don't agree. For a many-electron system, one-electron wavefunctions
 are BY DEFINITION physically unreal. They can only be defined by
 neglecting certain very physical aspects, corresponding to a model
 where electron correlation effects are neglected. According to basic
 Physics, real electrons instantaneously correlate their individual
 movements, but electrons in orbitals don't. Electrons in orbitals are
 'quasi-particles', not 'real particles'.
 > As J S-L points out, MOs are one-electron functions; does this mean
 > that for hydrogenlike atoms they _do_ correspond to physical reality?
 For an isolated hydrogen atom, the orbital wavefunctions, using the
 reduced mass and relativistic quantum mechanics, probably corresponds
 closely to what one might choose to consider as 'reality'. But in the
 end, the question becomes entirely philosophical; for example, no-one
 has ever observed an isolated atom.
 > If MOs have no physical reality for multielectron species, why (a) is
 > Koopmans' theorem useful, why (b) do photoelectron spectra match the
 > predictions of MO energy-level diagrams, and why (c) does the Hueckel
 > 4n+2 rule, which is based on MO diagrams, work? Of course, it is
 > probably possible to formulate an MO-free electronic molecular theory
 > that leads to the same predictions a-c, but I suspect that in some
 > sense (but what sense?) MOs exist--occupied MOs; the meaning of a
 > virtual MO is harder to see.
 The MO concept is very useful, forming the basis for excellent models
 of chemical and spectroscopic behaviour. For example, if you adopt
 Koopmans' approximation (neglect of electronic correlation and
 reorganization effects on ionization), Koopmans' well-known theorem
 applies. This is frequently a very good model, largely because the
 different errors introduces by the adoption of Koopmans'
 approximation tend to cancel each other out. But sometimes Koopmans'
 approximation is a bad approximation, and Koopmans' theorem does not
 apply. Or in the other words: The MO picture of ionization is
 sometimes a good model, and sometimes it is not.
 In any case: MOs don't exist in the physical sense. But of course,
 you may say that they 'exist' in our minds!
 Jens >--<
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