[Ifeffit] Error in running Artemis - Rmin too smaller than Rbkg

Matt Newville newville at cars.uchicago.edu
Tue Sep 8 06:30:14 CDT 2015


On Mon, Sep 7, 2015 at 9:53 PM, M.F.Bertuzzo <mb733 at kent.ac.uk> wrote:

> Dear all,
> I am facing this error when I try to run a fit. The file in question
> required a larger than normal Rbkg value of 3.0.
In what sense did this **require** Rbkg > 3.0?    Normally (and for your
data as well), Rbkg around 1.0 is sufficient.   Well, if you want to
analyze the first shell, that is.    Your example nicely demonstrates how
Rbkg > first shell distance can remove the first shell from the EXAFS... if
that's what you want.

> I have attatched the Athena file & the subsequent Artemis project file
> that I tried to fit so you can see the error I am facing.
> >From what I can understand, I need to reduce the Rbkg value in the Athena
> file to a lower number. But whenever I try to do this it causes the
> background function to mess up which then also causes problems for the fit
> in 'R' space.
Well, it needs to be a lower number if you want to analyze the first shell,
because with Rbkg=3, you've mostly removed the first shell from your data.

> Is this problem because I have badly modeled the data badly in Athena to
> begin?
> I'm not really sure how to solve the problem so your help and advice is
> much appreciated!
For your data, I'd recommend trying kweight=2 in the background
subtraction, and perhaps moving e0 up a little bit.  You could try a
"clamp" on the **high k**  side, too.    Be aware that you're not going to
analyze the data below 2 Ang^-1 anyway, so that if the background looks
like it's way to high at very low k, this is not actually a problem.

Hope that helps,

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