# [Ifeffit] MoS2 fit

Kelly, Shelly D. SKelly at anl.gov
Tue Aug 17 13:38:21 CDT 2004

```Hi Dan,

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
values.

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.

HTH
Shelly

> -----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
>
>
>
>
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```