[Ifeffit] FEFF report: Hard tests failed in fovrg.

Naumova, Maria maria.naumova at desy.de
Tue Aug 6 10:55:57 CDT 2013

Dear EXAFS people,

I have a question that appeared in FEFF and IFEFFIT several times. Still I didn't get an answer.

Here are some links to this question discussed or asked previously:

I have EXAFS of several similar organic complexes - copper which has S, N and Hal (Cl or I), and sometimes copper in its first coordination shell and more C and N in ligands.
I have structural data for them which I use as a model for FEFF.
When I run FEFF via Artemis I get the following report for all my substances (I have 6 potentials in the list): 

Calculating potentials and phases...
     free atom potential and density for atom type    0
     free atom potential and density for atom type    1
...and so on...
     overlapped potential and density for unique potential    0
     overlapped potential and density for unique potential    1
...and so on...
     muffin tin radii and interstitial parameters
     phase shifts for unique potential    0
   Hard tests failed in fovrg.
  Muffin-tin radius may be too large; coordination number too small.
     phase shifts for unique potential    1
   Hard tests failed in fovrg.
  Muffin-tin radius may be too large; coordination number too small.
...and so on...
 Feff done.  Have a nice day.
In FEFF documentation I read the following:

"To save time the code calculates the overlapped atom potential for each unique potential only once, using as a sample geometry the first atom in the atom list with a given unique potential index. Thus it is essential that the neighborhood of that sample atom be representative. Failure to do so may cause the code to perform poorly. " http://leonardo.phys.washington.edu/feff/Docs/feff6/feff6-4.html#pot

I think that at least for copper and halogen or sulfur geometry of every atom in the atom list is representative.

"Often we receive reports by users of older versions of FEFF of bugs that are fixed in the latest releases. Other code failures can often be traced to input file errors, sometimes quite subtle. An example would be non-physical widely spaced distributions of atoms. Symptoms of this common problem are very large muffin-tin radii (see the header of any .dat file) and possibly a failure of the phase-shift program to converge."
http://millenia.cars.aps.anl.gov/pipermail/ifeffit/2005-October/002080.html Here the reason is explained.

Since it's an organic complex local distances between neighrours are small, but distances between parts of molecule can be quite big. Still it's not "non-physical widely spaced distributions of atoms". For example distance from copper to the first shell is 2 - 2.3 A (2.6-2.7 to I and Cu). To the "second shell" (ligand atoms connected to the first shell atoms or not connected but close to central atom) - 2.9 - 3.4 A.

Finally what I want to know:

May I use this FEFF calculation as a valid base for future fit? Or this error means that FEFF doesn't work correct and I can't rely on its output? If the phase-shift program failed to converge does it mean that it could stop in some completely unrealistic result?

I attach several FEFF inp files which have this problem.

Thanks for your help,

Maria Naumova
PhD student in DESY
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