Re: CCL:Orbitals

2003 May 28
 Hello,
 Good points! Especially the idea that MOs are unreal _by definition_. If you
 insist that an MO must be a one-electron function (I know this is in the usual
 definition) then I suppose it could be "real" only for a hydrogenlike
 atom. For
 these, as you imply, the exact solutions of the Schroedinger equation evidently
 are physically real. As for the point about observing an isolated
 system--Heisenberg and Platonic reality....that could lead one far afield,
 perhaps.
 EL
 ==================
 Jens Spanget-Larsen wrote:
 > 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 >--<
 >
 > =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
 > JENS SPANGET-LARSEN         Office:         +45 4674 2710
 > Department of Chemistry     Fax:            +45 4674 3011
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