[Ifeffit] MoS2 fit
Kelly, Shelly D.
SKelly at anl.gov
Tue Aug 17 13:38:21 CDT 2004
Why are you using only one delr value and two dE values?
I would suggest that you try it the other way around.
two or more delr values with only one dE value.
When you try the different models compare the reduced-chi-square values
as you are doing but also
look at the Backfourier transfrom of the data and the fit - or the
chi(k) data if it is clean. Are they crossing zero at the same places.
Do they miss more at low q or high q? Missing at low q/k would suggest
separate dE values while missing at high q/k would suggest separate delr
Would your model be better described by using an overall
expansion/contraction term rather than one delr value?
Using one delr means that all the paths move by -0.01 angstroms. Using
one expansion term would allow all paths to contract by 0.1% which
might be more reasonable. So that the long paths shrink in more than
the short paths.
To use a expansion term use the special "reff" variable which stands for
the "effective r" i.e. the effective half path length.
set delr1 = alpha*reff
set delr2 = alpha*reff
guess alpha= -0.01
-yes please drop the third cummulant.
> -----Original Message-----
> From: dmc at pdx.edu [mailto:dmc at pdx.edu]
> Sent: Tuesday, August 17, 2004 1:24 PM
> To: XAFS Analysis using Ifeffit
> Subject: [Ifeffit] MoS2 fit
> Hi Mailing list,
> Well, the chief concern I have with my fit now is that
> I have some
> negative parameters (delr and e0_1):
> Guess parameters +/- uncertainties (initial guess):
> amp = 0.7709790 +/- 0.0631820
> e0 = 1.1165150 +/- 1.0100360
> delr = -0.0106440 +/- 0.0077110
> ss = 0.0032510 +/- 0.0005200
> e0_1 = -4.6848070 +/- 1.7781210
> third = 0.0000370 +/- 0.0001070
> So, I guess I can drop the third cumulant.
> This is a fit using 6 paths (1,2,3,6 are single scatterers)
> These are the results this time around:
> Reduced Chi-square = 410.285252396
> R-factor = 0.014599348
> As you can see I am still using two e0's, it is still
> dramatically improving
> the fit. Without: Reduced Chi-square = 1133.485950636
> R-factor = 0.044551155
> any advice welcomed, Dan
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