[Ifeffit] questions

Bruce Ravel ravel at phys.washington.edu
Mon Oct 27 14:35:34 CST 2003

On Monday 27 October 2003 03:10 pm, J. M. An wrote:
> Dear IFEFFIT helper,
> I am a novice of IFEFFIT and would like to ask some questions about 
> fitting variables used in the program.
> I've been trying to fit my FEFF 8 calculation result to an experimental 
> data for a bulk compound CdSe.
> Essentially, the fitting variables, S02 and DW factor (the correlated 
> Debye model) for the first-shell,
> are used for the fit. I can manage to fit quite nicely with those 
> variables, but the problem is that I want to use the values
> of the resulting fitting variables as input for CdSe quantum dot 
> calculations.  
> Normally, S02 and Debye keywords in FEFF are global paramaters, not 
> restricted to a single shell, so there seems to be
> no way that I can put the fitting values obtained from the fit into new 
> FEFF calculations.  
> I was wondering if someone has experience to use bulk fitting values as 
> input in a new FEFF calculation.

I don't think I quite understand what you want to do, but here are a
couple pieces of advice:

1.  It is my strong preference never to introduce S02 or sigma^2 into
    the feff calculation when I am doing exafs analysis.  It is much
    easier and much less confusing to use S02=1.0 and all sigma^2=0 in
    the feff calculation.  That way, any values that you get for those
    parameters from a fit using ifeffit are absolute numbers and do
    not require that you remember what you did in feff in order to
    interpret the ifeffit result.

2.  I do think that redoing the feff calculation might be useful at
    some time during the fit, but only if I have reason to believe
    that the *structural* parameters used in the feff calculation were
    in error relative to the real material.  If the distance between
    atoms in the real material are quite different from what you used
    in the feff calculation, then you might worry that the phase
    shifts and scattering amplitudes are not as good as they might be.

Given all that, I am not sure what you hope to accomplish by your new
feff calculation.  

Hope that helps,
 Bruce Ravel  ----------------------------------- ravel at phys.washington.edu
 Code 6134, Building 3, Room 222
 Naval Research Laboratory                          phone: (1) 202 767 5947
 Washington DC 20375, USA                             fax: (1) 202 767 1697

 NRL Synchrotron Radiation Consortium (NRL-SRC)
 Beamlines X11a, X11b, X23b
 National Synchrotron Light Source
 Brookhaven National Laboratory, Upton, NY 11973

 My homepage:    http://feff.phys.washington.edu/~ravel 
 EXAFS software: http://feff.phys.washington.edu/~ravel/software/exafs/

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