<excerpt>Hi Dan, At 12:58 PM 8/16/2004 -0700, you wrote:

Scott--

Ok, I'll put it on the mailing list from now on, but I used a different model

from the ICSD and it worked muc better. I used the hexagonal model and this is

the results:

Reduced Chi-square = 382.556284248

R-factor = 0.015036203

Do you use these to determine if the fit "doesn't work"?

The improvement in the stats is suggestive that the hexagonal model may be better than the trigonal model. As a rule of thumb, R-factors of below 0.02 are generally indicative of a <italic>statistically </italic>good fit (there may still be other reasons to doubt it!) and between 0.02 and 0.05 is indicative of some problems with the model (perhaps how the parameters are constrained or impurities), but is still on the right track (I've published at that level). The reduced chi-square is nearly worthless as an absolute measure, because of the necessity for having a good estimate of the measurement error. Reduced chi-square <italic>is</italic> useful in the way you're already using it, though...for comparing two fits of the same data.

About separation of the three paths they are here:

# degen reff amp fs scattering path

0001 6 2.414 100.00 : [+] S_1 [+]

0002 6 3.150 59.25 : [+] Mo_1 [+]

0003 6 3.968 21.22 : [+] S_3 [+]

What is fitting a third cumulant?

You'll see it as a path parameter in Artemis under "third." It is a measure of the asymmetry of the absorber scatterer distance: a positive third cumulant means it is more likely to find the absorber and scatterer a large distance above the mean than a large distance below the mean. It is sometimes helfpul to fit a third cumulant for nearest-neighbors because they are bonded; it is less appropriate for further shells, because the atoms move more independetly.

The two e0's really seems to improve the reduced chi-square.

The two ss's increased the reduced chi-square.

Hmmm...I don't know how you're grouping them between the three paths. I'd try one ss for the nearest-neighbor, and one for the next two paths. You might be doing one for Mo and one for S.

Thanks, Dan

--Scott Calvin Sarah Lawrence College

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