[Ifeffit] amplitude parameter S02 larger than 1

Bruce Ravel bravel at bnl.gov
Fri Mar 20 13:11:58 CDT 2015

On 03/20/2015 01:57 PM, huyanyun at physics.utoronto.ca wrote:
> I feel happy that you still remember that bizarre system which I am
> still working on.  Your brought up a very interesting test to do. I will
> definitely do this test experiment to see what would happen to the
> amplitude S02 if the first-shell distance is larger than 3 angstrom.
> This might take me for a while. I will come back to you after I do it

Well, I've reviewed your CLS proposals at least twice and discussed
your problem with you at an XAFS course.  Eventually things do stick
even in my tiny, little brain :)

On this page of the Artemis manual:


I explain why the Artemis user should be careful not to misuse the
quick first shell tool in Artemis.  The bottom line is that unphysical
muffin tin radii in the Feff calculation show up in large part as
incorrect amplitudes of the calculated chi(k) for the path, as you can
see in the second and third figures on that page.  In fact,
unphysically large muffin tins lead to larger amplitudes, as shown  by
the red lines

It's a little hard to decide exactly how your physical near-neighbor
separation of 3.3 A should be related to a Feff calculation that
purposefully sets the muffin tins to be loo large, but I think you can
see why I'm concerned that this effect might be related to your large
fitted S02 values.


  Bruce Ravel  ------------------------------------ bravel at bnl.gov

  National Institute of Standards and Technology
  Synchrotron Science Group at NSLS-II
  Building 535A
  Upton NY, 11973

  Homepage:    http://bruceravel.github.io/home/
  Software:    https://github.com/bruceravel
  Demeter:     http://bruceravel.github.io/demeter/

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