[Ifeffit] SCF and FMS radius

Bruce Ravel bravel at bnl.gov
Mon Jan 12 08:34:20 CST 2009

On Sunday 11 January 2009 12:23:26 pm Eugenio Otal wrote:
> I am simulating the XANES spectra of a erbium atom in an interstitial site
> of ZnO. I needed a SCF radius of 9.75 (316 atoms) and FMS radius of 11.90
> (599 atoms) to obtain no change in the simulated spectra (differences of
> 0.01% or less)
> Now I am trying to simulate Er(OH)3 and it is using SCF 10.90 (622 atoms)
> and I still haven found convergence, I will need more even larger radius
> for FMS.
> Does the XAS phenomena have this distances? Has this distances physical
> meaning?


The mean free path is quite long just above the Fermi energy.  It is 
conceivable that, in a well ordered material, the cluster size required for 
convergence can be quite large.  I believe that a very large cluster -- 
something like 13 A -- is needed for pure silicon.  Of course, I doubt that 
erbium hydroxide is that well-ordered!

So, are you are seeing physics associated with the mean free path or a problem 
in Feff's construction of the potentials?  I'm afraid I don't know how to 
answer that, but hopefully John or one of his gang will weigh in.  Do you see 
the changes larger than your threshold over the entire data range or only 
very close to the edge?

Good luck,


 Bruce Ravel  ------------------------------------ bravel at bnl.gov

 National Institute of Standards and Technology
 Synchrotron Methods Group at NSLS --- Beamlines U7A, X24A, X23A2
 Building 535A
 Upton NY, 11973

 My homepage:    http://xafs.org/BruceRavel
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