[Wien] Wien vs. FEFF

[fecher]

Sorry, it seems to me that you are a little off topic, don't you think you are 
in the wrong forum.
There should be a way that you communicate with each other directly by e-mail 
and leave this place for hard Wien topics.

Ciao Gerhard

Am Montag, 17. Januar 2005 17:10 schrieb Michel Jaouen:
> 	Dear Wien Users,
>
> 	I will add few comments to the following contributions.
>
> >I'd like to make an addition to the letter of Kevin:
> >FEFF is better for higher energies above the Fermi energy, as it is not
> >linearized method (as WIEN), and the multiple-scattering methods are good
> > for higher energies as a whole. The larger cluster you take in FEFF the
> > closer you may come down to the E_F. If you are interested in more than
> > 30-40 eV above E_F, you should use FEFF, in a closer region WIEN is
> > better.
>
> I quite generally agree with the above remark. Feff is mainly devoted
> to spectroscopies (xas, eels, xes) and in many cases it gives ldos
> that are very close to those obtained with Wien or any other band
> structure codes when the muffin-tin approximation can be considered
> as acceptable (metals or metallic alloys). It can also work for oxide
> or covalent materials in some case (I have made nice diamond or O-K
> edge for STO) but you then have to play with the cluster size and the
> angular momentum expansion (for instance for diamond you need to use
> 490 atoms and an s-p-d basis set to obtain all the xanes fine
> structures excepted the exciton line). Futhermore you can use the
> tdlda that may be of help in some case (L edges for instance) even if
> it is not yet fully automated.
>
> >  > There are also a lot of effects which neither of the programs get
> >  > right :
> >>
> >>  wien is ground-state, and feff (currently) only has one-electron
> >> Green's functions; that's not good enough to treat excited states
> >> accurately (think of core hole effects, etc.)
> >
> >In FEFF there is an account of the self-energy in the Hedin-Lundqvist
> >approximation in order to go beyond the DFT theory, though when I did this
> >for the NiO case I didn't obtain an improvement of the resulting EELS
> >spectra.
>
> You can also play with other exchange for screening (Dirac-Hara,
> ground state (that must be used to compare the ldos with those
> calculated from Wien for example, partially local, ..). You have also
> to take care of 2 points when comparing a feff calculation to eels
> measurements: i) the background is not the same in eels ans xas and
> you have to correct for, ii) quite generally elnes spectra are
> different than xanes ones due to the lower energy resolution in the
> first case (I have never seen a routine diamond elnes spectrum
> similar to the xanes one) and to the effect (not already  fully
> understood) of the swift electron in eels which "distort" the atomic
> wavefunctions (a kind of polarization effect) and thus modifies the
> shape of the spectrum at the edge jump.
>
> >The FEFF accounts the dipole transitions only, that is, it is valid for
> > XAFS (XANES), and not EELS; though very often the EELS spectra have
> > rather low contribution from nondipole transitions.
>
> I want to correct this which is not true: you can use the MULTIPOLE
> card. Default is only dipole, but you can calculate dipole and
> quadrupole or dipole and magnetic dipole transitions (only with
> feff8).
>
> 	To summarize, the main drawback of feff is that it is
> restricted to the muffin-tin approximation (a full potential version
> is under developpment), but it is a nice tool (fully automated) that
> is complementary to a code like Wien. It depends of which physics you
> are interested with: if it is band structure, use Wien, if it is
> spectroscopy, use feff.
>
> 	I hope it may help
>
> 	Best regards
>
> 	Michel Jaouen