Re: CORRECTIONS card

["John J. Rehr" <jjr@leonardo.phys.washington.edu>]

Dear Sergio,
 
> Please let me make you a "practical" question. I am simulating XANES
> spectra and I use the card CORRECTIONS to match the experimental threshold, 
> but some times happened that a simulation that gives a fine structure in 
> good agreement with the experimental data changes after the use of the card 
> CORRECTIONS, giving a different result.
> My two questions are: why is happening this?, and what do you use to do in 
> this situations?, just make a simple translation of the simulated spectrum?.

  I assume you are referring to Fermi energy shifts, not absolute
energy shifts. The CORRECTIONS card can't shift absolute x-ray energy
values - you have to do that by hand.

  The CORRECTIONS card only shifts the photoelectron kinetic energy
edge (more precisely the Fermi energy) and broadening. You choose
Vr by matchine peaks in the near edge structure and then fixing Vr
to get the edge right. Often the experimental spectra has more broadening
than FEFF. You can add broadening by rerunning the last module with
an additional imaginary potential Vi.

  Finally the CORRECTIONS card is not as reliable as the EXCHANGE
card in adjusting the spectra since it only adjusts the Fermi
energy, while the EXCHANGE card also changes the phase shifts.
We've improved the algorithm in FEFF8.2, but you should check
the spectra both ways to be confident about the results. Please
let us know if the results do differ substantially however.

  Sincerely,
  John Rehr