Matt, Thanks for the explanation. In this case, next two questions. 1. Is there a way to say it was rejected other than re-running feff with CRITERIA 0 0? I expected it to show in Feff.run file with "neglected" flag. My "default" Feff.run file says " Curved wave chi amplitude ratio 4.00% Discard feff.dat for paths with cw ratio < 2.67%" and the "neglected" flag is present for those with <2.67%. 2. Also there is still one path left (all paths No# 10 Se-Zn-Se) with cw ratio 2.83, which passes the <2.67 criterion but is weaker than the two paths that are rejected before it with amps 3.952 and 3.999. In other words - when using "default" generated feff.inp, which paths in Feff.run are reported as neglected and which are not reported at all? Best Pawel

...

Message: 1 Date: Thu, 28 Jul 2011 18:25:56 -0400 From: Pawe? Zajdel To: Subject: [Ifeffit] FEFF6L.02 - missing path? Message-ID: <5EB6FA326A714E6BAD00215CBFB556AE@NewLapek> Content-Type: text/plain; charset="iso-8859-2"; Format="flowed"

Dear All,

I am facing a strange issue while playing with EXAFS for a spinel type structure. I tried two typical examples: MgAl2O4 and ZnCr2Se4 with octahedral site K edge of Al or Cr. The MgAl2O4 is fine but for ZnCr2Se4, FEFF seems to omit one scattering path. It is not reported as being rejected by <2.6% amp criterion, so I wonder, while it is not shown. This is path 3 (as seen in MgAl2O4), which goes from Cr/Al to anion, then over the "octahedron" edge to another anion and back to Cr/Al. For oxide Reff=3.2251 <cut from feff.run> path cw ratio deg nleg reff 1 100.000 6.000 2 1.9371 2 37.913 6.000 2 2.8853 3 10.687 12.000 3 3.2251 <-- It's here 4 9.664 2.000 2 3.3444 </cut> For selenide Reff=4.2354 <cut from feff.run> path cw ratio deg nleg reff 1 100.000 6.000 2 2.5248 2 47.173 6.000 2 3.7084 <-- should be after this one. 3 25.382 6.000 2 4.3485 </cut> I am attaching two atoms.inp files, which may help. It is space grup 227, standard setting (2), atom list is correct. I am using Ifeffit 1.2.11c with Feff 6L.02. It s present under WinVista and Linux on two different machines. Am I missing something? I would appreciate any suggestions.

Best Pawel

Message: 2 Date: Thu, 28 Jul 2011 21:27:57 -0500 From: Matt Newville To: XAFS Analysis using Ifeffit Subject: Re: [Ifeffit] FEFF6L.02 - missing path? Message-ID: Content-Type: text/plain; charset=UTF-8

Pawel,

For these two structures, the location of the anions (Se for ZnCr2Se4 and O for MgAl2O4) are not in in exactly the same location in the unit cell. The nearly identical double scattering paths: Al->O->O->Al and Cr->Se->Se->Cr each have small estimated scattering amplitudes ("Zabinsky Curved Wave Importance Factors"). The Cr->Se->Se->Cr path is filtered out with the default settings, and so not included in the path list, while the Al->O->O->Al path is not filtered out.

You can guarantee to see all the paths if you put CRITERIA 0 0

in the feff.inp file.

For a more detailed reason of why Al->O->O->Al has an estimated amplitude of 10.7 (and so not filtered out) while the Cr->Se->Se->Cr has an estimated amplitude of 3.9 (and so is filtered out), there are a few points to consider:

1. The amplitude reported is relative to the strongest scattering path, which always has amplitude 100. In both cases, this is the first shell Al->O->Al and Cr->Se->Cr. Se is a stronger back-scatterer than O.

2. Both absorber and scatterer are heavier in the Zn-Cr-Se case than the Mg-Al-O case, and the the heavier atoms are more point-like in their electron density.

3. The Se atoms are closer to 0.25, 0.25, 0.25 in the unit cell than the O atoms. This makes the Cr->Se->Se->Cr path have a scattering angle beta a little closer to 90 degrees, which gives less scattering amplitude.

I think all three of these conspire to give the Cr->Se->Se->Cr path less amplitude than the Al->O->O->Al path.

Hope that helps,

--Matt

-- dr Pawel Zajdel Division of Physics of Crystals Institute of Physics University of Silesia Uniwersytecka 4 40-007 Katowice Poland [48]323591978