[Ifeffit] Amplitude in Artemis

[Bruce Ravel]

An additional US$0.02 worth: The linear combination fits tend to be
pretty simple, well defined, and well constrained in a statistical
sense.  If you get one of those scary sounding messages (with the
scary colored letters ;-) it is usually telling you something about
your fit.  Hopefully, that something is not that the program is
broken, that your standards are broken, or that you aren't using the
program correctly.  Rather, it should be telling you something about
your choice of standards.

If you have taken care to normalize your data well and consistently,
then a fit that pins a weight at zero probably means that that
standard does not exist in the sample.  Thus, the warning is intended
to be useful information.

Athena's LCF utility has a lot of bells and whistles, some of which
Dave mentions.  The point of having all those knobs to turn is to be
able to poke corners of the fit and to try to understand why a fit is
turning out the way it is turning out.  You should not hesitate to
turn those knobs -- they aften provide useful information.  Everything
is explained on the document page for the LCF utility, albeit
tersely.  If the effect of any of the knobs is unclear, don't hesitate
to ask the list.

B

On Thursday 02 March 2006 16:21, bsin1033 at mail.usyd.edu.au wrote:
> Dave,
>
> Thank you for your comments. By doing your way it works but one of the
> standard is not quite right, as you have also suggested. We will rerun the
> standards and try it again.
>
> Cheers,
>
> Balwant
>
> Quoting "Barton, David (DG)" <DGBarton at dow.com>:
> > Balwant,
> >
> > I have done a number of LCF using Athena and have found that this error
> > message is typically associated with the rejection of one of your
> > standards as a possible component in the fit.  This error message may be
> > avoided in most cases if you uncheck the options of --weights between 0
> > &1 and --Force weights to sum to 1 AND you put in an appropriate initial
> > guess for the fit.  When you rerun the fit with these options unchecked,
> > it should be obvious what caused the error since one or more of your
> > standard spectra will get a negative weight.  When this occurs it is
> > probably a good sign that the standard is not appropriate in the fit and
> > some other standard should be used.
> >
> > Hope this helps,
> > Dave
> >
> > > -----Original Message-----
> > > From: ifeffit-bounces at millenia.cars.aps.anl.gov
> > > [mailto:ifeffit-bounces at millenia.cars.aps.anl.gov] On Behalf
> > > Of bsin1033 at mail.usyd.edu.au
> > > Sent: Thursday, March 02, 2006 3:41 PM
> > > To: XAFS Analysis using Ifeffit; Kelly, Shelly D.
> > > Cc: XAFS Analysis using Ifeffit
> > > Subject: RE: [Ifeffit] Amplitude in Artemis
> > >
> > >
> > > Hi Ifeffitters,
> > >
> > > I was trying to use the linear combination fitting function
> > > in Athena to quantifying Cr(VI) and Cr(III) species. I used
> > > two endmembers of Cr species as the two standards but the
> > > program fails and gives error message "The fit flagged a
> > > warning--probably that error bars could not be calculated.
> > > That is probably an indication that one or more of your
> > > standards are inappropriate for this fit." It appears I am
> > > either using the wrong settings or even wrong function to do
> > > the job, any suggestions. We have additional 3 spectra with
> > > variable combinations of Cr(VI) and
> > > Cr(III) as well.
> > >
> > > Cheers,
> > >
> > > Balwant Singh
> > >
> > > ----------------------------------------------------------------
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-- 
 Bruce Ravel  ---------------------------------------------- bravel at anl.gov

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 MRCAT, Sector 10, Advanced Photon Source, Building 433, Room B007

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