[Wien] Wien vs. FEFF

[Lyudmila Dobysheva]

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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