[Ifeffit] Use of the 3rd and 4th Parameter

[Scott Calvin]

Hi Michael,

It is best to start by guessing the third cumulant only, leaving the fourth cumulant set to zero. A survey of the published literature shows very few examples where the fourth cumulant has been fit--by that point the cumulant expansion itself is starting to become iffy, which is not the case when you just have "some" disorder.

Guess the third cumulant as zero, and then observe the best-fit value and its uncertainty. A typical result might be something like 0.0005 +/- 0.0002 angstroms cubed. That result would suggest that the third cumulant was nonzero, as the bestfit value is outside the reported uncertainty. Comparing the reduced chi-square parameter with the third cumulant set to zero and guessed will give you a sense of whether it resulted in a statistical improvement to the fit; using the Hamilton test would be even better.

The third cumulant can be positive, negative, or zero.

To get a sense of how large the magnitude can be, think about the units: angstroms cubed. Very roughly, we usually expect delr to be less than 0.1 angstroms, sigma2 to be less than 0.01 angstroms, and the third cumulant to be less than 0.001 angstroms. If we're getting numbers bigger than that, it suggests that a different starting model might be called for, rather than trying to use a cumulant expansion. You certainly should never guess the third cumulant at 0.5--that's way too big!

--Scott Calvin
Sarah Lawrence College


On Jun 16, 2014, at 11:15 AM, Michael Kömm <michael.koemm at uni-jena.de> wrote:

> Hello,
> 
> I'm trying to make an EXAFS analysis on the Structure of Tb in a 
> borogermanate glass System. The reference is an .cif of 
> Terbium-Gallium-Garnet (TGG).
> 
> I expect some disorder caused to working in glass. Therefore I would 
> like to use the 3rd and/or the 4th parameter. I tried to set a guess 
> parameter to evaluate those parameters. Could you tell me a reasonable 
> value for these parameters? The 3rd only varies a little bit around its 
> guessed value (+- 0.01) and if I try to do the same with the 4th 
> parameter no fit at all takes place. The errors in the GDS-window are 
> exactly 0.00000 with no fit taking place (enot = 0, amp = 0 (guess amp = 
> 1) delr = 0 and ss = 0.003) and the R-factor of the fit is way over the 
> top (>10^20).
> 
> I've chosen a value of 0.5 for the 3rd and the 4th parameter just to see 
> if they're working and I was only doing a quick first shell fit with the 
> surrounding oxygen-shell.
> 
> Please can somebody tell me how to deal with this problems.
> 
> Thanks a lot for your time and efford.
> 
> Sincerly yours,
> 
> Michael Kömm
> 
> 
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