[Wien] binding energies

[Gilles Hug]

Hi,
I quite fully agree with Gerhard.
Just a few more words for completeness.
When I calculated excitation energy for a 1s electron (for X-rat
absorption for example) I would make the difference in total energy
between ground state and a "static" excited state. This would a kind of
first order approximation. Then Wien gives a very good answer, say within
5-10 eV. You mignt not be happy with it but considering it is a difference
between two very large numbers, the accuracy is not so bad.
On the other side FEFF is a kind of GW formalism which goes beyond ground
state. However, although it has a built in fully relaxed core hole
approximation, it is NOT a "full potential". It is meant to
calculate the
shape of the fine structure, their intensities and relative energy
positions but NOT their absolute positions. The
absolute position is calculated from a purely atomic calculation. Therfore
it is usual to shift by a few eV or so the
position of the spectral features. This is not harmfull and is documented
in FEFF's UG (I think).
So beware that you may have "accidental" good match. It will not be always
the same.
(Un?)fortunately, as Gerhard said, their is still some work ahead for
theorists...
Joyeux Noel
Gilles

On Thu, 16 Dec 2004, fecher wrote:

> The answer is simple, XPS does not measure the ground state binding energies.
>
> This is not just the problem of the N to N-1 electron transition alone (in the
> sense of calculating just the two cases, ground and excited), but will also
> depend , to say in not too physical words, on "secondary" effects some known
> as "multiplett" effects, some as "shake up",  "shake down", etc. as found in
> textbooks of corelevel photoemission (see.: e.g. reviewing articles by Chuck
> S. Fadley and others). The results of calculations may strongly depend
> whether one uses sudden approximation or adiabatic approximation.
> Calculational results may also strongly depend how sorrowly the transition
> matrix elements are calculated.
>
> With FEFF, the result will depend which potential you use and this is most
> probably not the same like the one from Wien2k, so it is no wonder that the
> energies and trends will be different.
>
> The problem of comparing calculated and measured photoemission spectra is, to
> my knowledge, not completely attacked for the most general case, as this is a
> very complicated many-body problem. However, FEFF seems to be a very advanced
> one.
>
>
> Ciao Gerhard
>
>
> Am Donnerstag, 16. Dezember 2004 00:37 schrieb Vicki Keast:
> > Dear fellow Wien2k users,
> >
> > I am trying to compare experimental (XPS) binding energies to
> > calculations for  S atoms in different Ni sulfides with different
> > structures and co-ordination.
> >
> > The core state energies calculated by Wien are quite different from
> > experiment and FEFF8 calculations. I assume this is because the zero in
> > the Wien calculations is the average Coulomb potential not the the
> > vacuum level. We are also seeing that the trends in the core level
> > energies are different for Wien than experiment and FEFF.
> >
> > Does anyone have any advice on how to go about comparing calculated and
> > experimental binding energies and also on how reliable we might expect
> > them to be?
> >
> > Thanks and best wishes to you all,
> >
> > Regards
> > Vicki
>
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