[Ifeffit] delR and delE

[Kelly, Shelly D.]

See below for clarification..



> -----Original Message-----
> From: Kelly, Shelly D. 
> Sent: Wednesday, October 13, 2004 3:10 PM
> To: XAFS Analysis using Ifeffit
> Subject: RE: [Ifeffit] delR and delE
> 
> 
> Ritika,
> 
> delr and Eo are correlated.  So you can (1) force delr to the 
> same for two paths with different Eo-values, or (2) use two 
> different delr-values with one Eo-value.  Most prefer the 
> second choice over the second choice.  

This should read "Most prefer the second choice over the first."

> 
> Also, Eo is important at low k-values.  If you move your 
> k-range that you use in the FT of your data you might find a 
> different Eo value, or that only one Eo is needed with a 
> shorter k-range than with a longer k-range.
> 
> I do not think that you model is "okay" with three Eo values. 
>  How much does EO differ?  Usually Eo is the last button I 
> turn on when I try to make a model fit the data.  Large Ezero 
> values (> 10 eV) are common if you have a poor model.  So use 
> only one Eo at the beginning.  Often I just set it to zero, 
> and then when the model gets close, try to add one or two 
> more to really get it right on.
> 
> I would try only one Eo first. Let delr vary for all the 
> paths and change the k-range.  See how small the k-range 
> needs to be before you can fit the data with only one E0 value.
> 
> 1.9 Angstroms is a short distance.  It would seem more 
> reasonable to me that you might need two Eo values.  One for 
> this short shell and then one more for all the other shells.
> 
> Give these things a try and tell us how it goes.
> 
> Shelly
> 
> > -----Original Message-----
> > From: ritika.uppal at yale.edu [mailto:ritika.uppal at yale.edu]
> > Sent: Wednesday, October 13, 2004 2:50 PM
> > To: Segre, Carlo U.; XAFS Analysis using Ifeffit
> > Subject: Re: [Ifeffit] delR and delE
> > 
> > 
> > 
> > I have 3 different values of Eo for the fit. 2 for the first
> > shell and 
> > the 3rd for the rest of the paths. Actually the bond distance 
> > of the 6 
> > oxygens in the first shell differs. 3 of them have a bond length of 
> > around 1.9 A and the other 3 around 2.2 A. So using one value of Eo 
> > does not fit the second peak very well.
> > 
> > Ritika
> > 
> >  
> > Quoting "Carlo U. Segre" <segre at iit.edu>:
> > 
> > > 
> > > delR is just the deviation from the distance you used in
> > the feff.inp
> > > file to generate the paths.  It can be positive or negative
> > depending
> > > on
> > > 
> > > whether these atoms are closer or further away from the
> > central atom.
> > > 
> > > By delE, do you mean Eo?  When you say two values, do you mean
> > > different values for two different shells?
> > > Carlo
> > > 
> > > 
> > > On Wed, 13 Oct 2004, ritika.uppal at yale.edu wrote:
> > > 
> > > > Hi guys,
> > > > I have a few questions about the fitting parameters. 
> the delR for
> > > my
> > > > fit has a negative value. Is that wrong or its possible 
> to have a
> > > > negative value?? and also for the first shell, I have 2 
> values of
> > > del
> > > > E. Does that mean that the model I am using is not a good one??
> > > >
> > > > Thanks for all your help
> > > > Ritika
> > > > _______________________________________________
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> > > 
> > > --
> > > Carlo U. Segre -- Professor of Physics
> > > Associate Dean for Special Projects, Graduate College Illinois 
> > > Institute of Technology
> > > Voice: 312.567.3498            Fax: 312.567.3494
> > > Carlo.Segre at iit.edu    http://www.iit.edu/~segre
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