[Wien] Wien vs. FEFF

[Jorissen Kevin]

Hello Lyudmila,
thanks for your remarks.
 
One small counter-remark : feff can do quadrupole transitions as well (not only dipole).
 
About the size of the clusters for feff : I regularly find that the default limit of 87 atoms is unsufficient to get the details to converge; typically, 200 atoms in the cluster gives well converged results, I think.
 
A final remark about speed :
for extended fine structure (exafs/exelfs), or for small clusters (say up to 87 atoms), feff is fast (order of a few minutes for a calculation).  In wien, calculations are only that fast when the unit cell is rather small and when you do not account for the core hole effect at all.
For larger clusters in feff (say 200 - 300 atoms), the calculation of near edge structure (xanes/elnes) becomes slower (to the order of hours for a calculation), comparable to small core hole calculations in wien (but still faster than large super cell calculations in wien).
 
Generally, feff will be considerably faster.
On the other hand, a converged scf calculation in wien gives you access to a lot of additional properties.
 
 
 
 
Kevin Jorissen
 
EMAT - Electron Microscopy for Materials Science   (http://webhost.ua.ac.be/emat/)
Dept. of Physics
 
UA - Universiteit Antwerpen
Groenenborgerlaan 171
B-2020 Antwerpen
Belgium
 
tel  +32 3 2653249
fax + 32 3 2653257
e-mail kevin.jorissen at ua.ac.be
 

________________________________

Van: wien-admin at zeus.theochem.tuwien.ac.at namens Lyudmila Dobysheva
Verzonden: di 11-1-2005 8:25
Aan: wien at zeus.theochem.tuwien.ac.at
Onderwerp: Re: [Wien] Wien vs. FEFF



Dear Pedro,

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.

> 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.

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.

Best regards,
  Lyudmila Dobysheva
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