Re: [Ifeffit] [Attachment Removed] Re: Low amplitude chi signal of first shell calculated by FEFF

23 May 2023

      Hi Fan,

If you included an attachment, I did not see it.  If so, can you send it
some other way?

I would not expect that the Sr K-edge of SrTiO3 needs much playing around
with parameters in the feff.inp file prior to running Feff.   I would
advise against doing that unless you know what you are doing.  Don't set Vi
to be below 0.

I would be more inclined to believe that the data treatment had problems or
that the use of the feff paths had problems.   Without knowing what you
did, it is very hard to speculate what might be going wrong.

On Tue, May 23, 2023 at 9:38 PM Dongxiao  wrote:
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------------------------------
Hello, Matt, cc: who is interested,
Thank you very much for your reply.
The low amplitude chi signal is common when I calculated the A site EXAFS
signal of perovskite structure.
Especially the chi signal of first single scattering path (A-O) is
unreasonable low.
In my case (Sr K-edge EXAFS of SrTiO3), the intensity of calculated chi is
smaller than the low-temperature experimental chi.
I have been struggling with this problem for some time. I am looking
forward to somebody who can give me some advice..
I attached the related files for your check.
I try to vary Vr and Vi value and want to see how the amplitude change.
If set the Vi to negative, it indeed can increase the amplitude, but I
don’t know if it is proper.
Maybe the problem is caused by the bad muffin-tin potential, but I don’t
know how to improve it.
Can you give me some advice?
Best regards,
Fan
-----Original Message-----
Matt Newville
https://www.mail-archive.com/search?l=ifeffit@millenia.cars.aps.anl.gov&q=from:%22Matt+Newville%22
 Fri, 12 May 2023 14:25:36 -0700
https://www.mail-archive.com/search?l=ifeffit@millenia.cars.aps.anl.gov&q=date:20230512
Hi Fan,
On Fri, May 12, 2023 at 9:56 AM  wrote:
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Hi, All
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The problem I meet is the low amplitude chi signal of first shell
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calculated by FEFF.
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The consequence of the fitting is large value of SO2 (1.5-2.0), which is
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quite unacceptable.
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I tried to fit the dataset measured by different people at different
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beamline, the consequence is same.
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My data is T-dependent Sr K-edge EXAFS of SrTiO3, path file generated by
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FEFF10, fitting process by Larch+python.
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By check the email list, it seems the long shell distance (2.7 A) is the
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origin of the problem. I am not sure for that.
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I also try to vary the Vi value to improve the amplitude, the change is
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small.
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Can any body give me some advice?
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I would not expect that a Sr-O distance of 2.7 to 2.8Ang would cause an
amplitude problem.  I would tend to expect that being off by a factor of 2
would
imply normalization issues.    You could try to vary Vi, but I would not
guess that you need to do that.  Why do you set Vi to 2 (eV)?
Do you have data and/or analysis steps (script or Larch session file) you
can share?
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Best regards,
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Fan
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My feff.inp file is like:
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--------------------------------------------
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* This feff8 file was generated by Demeter 0.9.26
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* Demeter written by and copyright (c) Bruce Ravel, 2006-2019
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* --*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--
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* space = 221
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* a     =   3.90500    b    =   3.90500    c     =   3.90500
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* alpha =  90.00000    beta =  90.00000    gamma =  90.00000
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* rmax  =   9.00000    core  = Sr
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* polarization = 0  0  0
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* shift =   0  0  0
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* atoms
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* # el.     x           y           z        tag
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*   Sr     0.00000     0.00000     0.00000   Sr
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*   Ti     0.50000     0.50000     0.50000   Ti
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*   O      0.00000     0.50000     0.50000   O
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* --*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--
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* --*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--
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*  total mu*x=1:  32.298 microns,  unit edge step:  44.289 microns
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*  specific gravity:  5.118
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* --*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--
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*  normalization correction:    0.00029 ang^2
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* --*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--
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*  Sr K edge energy = 16105 eV
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EDGE K
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S02 1.0
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*         pot    xsph  fms   paths genfmt ff2chi
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CONTROL 1 1 1 1 1 1
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PRINT 1 0 0 0 0 3
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*         ixc  [ Vr  Vi ]     *** ixc=0 means to use Hedin-Lundqvist
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EXCHANGE 0 2 1 -1
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*** Radius for self-consistent pots (2 shells is a good choice)
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*         r_scf  [ l_scf   n_scf   ca ]   *** l_scf = 0 for a solid, 1 for
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a molecule
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SCF 7 0 100 0.1 1
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*         kmax   [ delta_k  delta_e ]     *** Upper limit of XANES
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calculation.
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* XANES     4.0
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*         r_fms     l_fms      *** Radius for Full Mult. Scatt. l_fms = 0
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for a solid, 1 for a molecule
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* FMS      16.77392  0
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*         emin  emax   eimag   *** Energy grid over which to calculate DOS
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functions
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* LDOS      -30   20     0.1
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*** for EXAFS: RPATH 5.0 and uncomment the EXAFS card
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CRITERIA 0 0
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* POLARIZATION  0   0   0
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COREHOLE FSR
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RPATH 7.0
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LDOS -30 20 0.1
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EXAFS 20.0
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--Matt

Matt Newville

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