[Ifeffit] FEFF6L.02 - missing path?

[Pawel Zajdel]

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 <pawel.zajdel at us.edu.pl>
> To: <ifeffit at millenia.cars.aps.anl.gov>
> Subject: [Ifeffit] FEFF6L.02 - missing path?
> Message-ID: <5EB6FA326A714E6BAD00215CBFB556AE at 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 <newville at cars.uchicago.edu>
> To: XAFS Analysis using Ifeffit <ifeffit at millenia.cars.aps.anl.gov>
> Subject: Re: [Ifeffit] FEFF6L.02 - missing path?
> Message-ID:
> <CA+7ESbpzXoP+OkH2Nq14DY-W4SP-cgCnh_0=+b7OhSvLMG80xA at mail.gmail.com>
> 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