[Ifeffit] Differences between Larch and Artemis when performing FEFF calculations

[Ava Rajh]

Dear all!

I haven't been able to find a similar question/issue in the previous 
threads, so I hope someone can help me figure out what is going on.

I am trying to fit a Carbon EXAFS spectra, using graphite as a model. I 
am first focusing on just the first two single scattering paths, so I 
calculated the theoretical paths with FEFF in Larch and tried using them 
to fit the spectra. I was consistently getting slightly lower distances 
than expected, but otherwise an OK fit.

The issue is, I tried to compare the analysis with a colleague who is 
using Athena. At first glance the EXAFS spectra, using the exact same 
parameters, looked very similar (but not exactly the exactly the same, 
this I attributed to Larch using a different autobk procedure). I would 
have however expected the theoretical paths to match exactly, if they 
were calculated and plotted with the same parameters. But they were also 
slightly different.  I then downloaded Athena and spent time trying to 
find where the differences come from. If I compare the first two 
calculated shells from Larch with the ones from Athena, with exactly the 
same set of test parameters (S02 = 1, E0 = 0, dr1 = 0, s2_1 = 0, dr2 = 
0, s2_2 = 0), the resulting models do not match. I made sure the paths 
are calculated from the came .cif file in both cases, use FEFF6, have 
the same calculated reference distances, same FT...

So, my main question is, am I missing something important in regards to 
calculations, why would the calculated paths be different and which one 
would be the "correct" one to use for the fit? And the other question 
would be about the fact that EXAFS spectra of experimental data look 
slightly different using Larch and Athena, am I right in disregarding 
this, or should I dig deeper and find the source of discrepancy?

I am enclosing a plot of just the calculated first two shells from 
Athena and Larch (FT: kmin =  2, kmax = 7.5, Fittting in R space, kw = 
3, kWindow = Hanning, dk = 1.0, Rmin = 0.6, Rmax = 2.1) along with the 
cif file I ended up using for testing the differences. If it would be 
helpful, I can also provide the project files and larch script I used 
for the dataset, but I am mainly interested in understanding the 
differences seen in the theoretical parts first. I tested this using 
Larch v 0.9.72 and Demeter 0.9.26

Thank you ver much for the help, and If I need to provide any additional 
info please let me know.
kind regards, Ava

-- 
Ava Rajh
-------------- next part --------------
A non-text attachment was scrubbed...
Name: Individual paths.png
Type: image/png
Size: 396985 bytes
Desc: not available
URL: <http://millenia.cars.aps.anl.gov/pipermail/ifeffit/attachments/20231016/df23d73b/attachment-0003.png>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: Combined paths.png
Type: image/png
Size: 322629 bytes
Desc: not available
URL: <http://millenia.cars.aps.anl.gov/pipermail/ifeffit/attachments/20231016/df23d73b/attachment-0004.png>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: EXAFS of measured data.png
Type: image/png
Size: 366077 bytes
Desc: not available
URL: <http://millenia.cars.aps.anl.gov/pipermail/ifeffit/attachments/20231016/df23d73b/attachment-0005.png>
-------------- next part --------------
An embedded and charset-unspecified text was scrubbed...
Name: C.cif
URL: <http://millenia.cars.aps.anl.gov/pipermail/ifeffit/attachments/20231016/df23d73b/attachment-0001.ksh>