[Ifeffit] Low amplitude chi signal of first shell calculated by FEFF

[Matt Newville]

Hi Fan,


On Fri, May 12, 2023 at 9:56 AM <dxfan at post.kek.jp> wrote:

> Hi, All
>
>
>
> The problem I meet is the low amplitude chi signal of first shell
> calculated by FEFF.
>
> The consequence of the fitting is large value of SO2 (1.5-2.0), which is
> quite unacceptable.
>
> I tried to fit the dataset measured by different people at different
> beamline, the consequence is same.
>
>
>
> My data is T-dependent Sr K-edge EXAFS of SrTiO3, path file generated by
> FEFF10, fitting process by Larch+python.
>
> By check the email list, it seems the long shell distance (2.7 A) is the
> origin of the problem. I am not sure for that.
>
> I also try to vary the Vi value to improve the amplitude, the change is
> small.
>
> Can any body give me some advice?
>

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?

>
>
> Best regards,
>
> Fan
>
>
>
> My feff.inp file is like:
>
> --------------------------------------------
>
> * This feff8 file was generated by Demeter 0.9.26
>
> * Demeter written by and copyright (c) Bruce Ravel, 2006-2019
>
> * --*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--
>
> * space = 221
>
> * a     =   3.90500    b    =   3.90500    c     =   3.90500
>
> * alpha =  90.00000    beta =  90.00000    gamma =  90.00000
>
> * rmax  =   9.00000    core  = Sr
>
> * polarization = 0  0  0
>
> * shift =   0  0  0
>
> * atoms
>
> * # el.     x           y           z        tag
>
> *   Sr     0.00000     0.00000     0.00000   Sr
>
> *   Ti     0.50000     0.50000     0.50000   Ti
>
> *   O      0.00000     0.50000     0.50000   O
>
> * --*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--
>
> * --*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--
>
> *  total mu*x=1:  32.298 microns,  unit edge step:  44.289 microns
>
> *  specific gravity:  5.118
>
> * --*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--
>
> *  normalization correction:    0.00029 ang^2
>
> * --*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--*--
>
> *  Sr K edge energy = 16105 eV
>
> EDGE K
>
> S02 1.0
>
>
>
> *         pot    xsph  fms   paths genfmt ff2chi
>
> CONTROL 1 1 1 1 1 1
>
>
>
> PRINT 1 0 0 0 0 3
>
>
>
> *         ixc  [ Vr  Vi ]     *** ixc=0 means to use Hedin-Lundqvist
>
> EXCHANGE 0 2 1 -1
>
> *** Radius for self-consistent pots (2 shells is a good choice)
>
> *         r_scf  [ l_scf   n_scf   ca ]   *** l_scf = 0 for a solid, 1 for
> a molecule
>
> SCF 7 0 100 0.1 1
>
> *         kmax   [ delta_k  delta_e ]     *** Upper limit of XANES
> calculation.
>
> * XANES     4.0
>
> *         r_fms     l_fms      *** Radius for Full Mult. Scatt. l_fms = 0
> for a solid, 1 for a molecule
>
> * FMS      16.77392  0
>
> *         emin  emax   eimag   *** Energy grid over which to calculate DOS
> functions
>
> * LDOS      -30   20     0.1
>
> *** for EXAFS: RPATH 5.0 and uncomment the EXAFS card
>
> CRITERIA 0 0
>
> * POLARIZATION  0   0   0
>
> COREHOLE FSR
>
> RPATH 7.0
>
> LDOS -30 20 0.1
>
> EXAFS 20.0
>
>
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-- 
--Matt Newville <newville at cars.uchicago.edu> 630-327-7411
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