Re: XANES again

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

Hi

 Here are some quick answers from my perspective, since these are general questions.
> 
> 1) Are there any "general" suggestions which might be useful (let's stick to 
> that Fe(OH3)-O-Fe(OH)2-O-Fe(OH)3 example again)? I found the hints in the 
> manual concerning the SCF and FMS card - but what, e.g. should one insert for 
> the SCF radius of the outer Fe atoms? Has FEFF problems in handling the fact 
> that the atom distribution around the outer atoms is so asymmetric?
   The radius should generally include a few coordination shells so
that all chemically relevant neighbors are included. More distant atoms have
small effects. My advice: try a larger radius to confirm that your calculation
is adequate.
> 
> 2) Concerning the equivalency of the O and H atoms:
> Should one do a distinct potential index for the different oxygen atoms or can 
> FEFF handle the bridging and endstanding O atoms in one potential index?
   Maybe (?). It's probably a good approximation for EXAFS, but sometimes
questionable for XANES. To check this, do a run with two potential indices and
see if you can tell the difference! I recommend doing checks for all aspects
of XANES calculations, e.g., the size of the cluster, lmax, etc., so you
have confidence in the results. It's especially important to be sure the
SCF calculation has converged for XANES calculations.
> 
> 3) Concerning the stochiometry which can be given in the potential section:
> For solids, index 0 should get 0.01 as stochiometry, the rest as defined by 
> the solid itself. For molecules, I just enter the stochiometry from the 
> sample (e.g. SF6 = 1 for S and 6 for F)? Does the stochiometry affect 
> something else than just the calculation of Ef?
  No. It's only used in potential construction.
  
  J. Rehr