[Ifeffit] [howto] configurational average in EXAFS with FEFF

[Bruce Ravel]

On Tuesday 27 March 2007 14:34, Mauro Rovezzi wrote:
> Anatoly,
>
> I don't quote your answer because I would like to add some more elements
> to specify what I'm searching for.
>
> In my previous post I have used the term "cluster" in a wrong way, that
> is I was thinking at the crystal-box (normally less than 216 atoms) used
> in the ab-initio density functional theory calculations and "doped" with
> few percent (< 10%) of an absorbing atoms. For example, if we put 5
> absorbing atoms in a box of 64 (that represent the crystal host) we have
> 5 possible configurations for our FEFF calculation and is impossible in
> an EXAFS fit to include all the important paths (too many!).
>
> My idea to take into account the numerical simulation and combine it
> with the experimental EXAFS data could be to use the pair radial
> distribution function extracted from the simulation, average the
> configurations, recalculate paths and proceed in the fit.

Mauro,

If I understand what you are trying to do, this paper by me might be
relevant: 

 Role of local disorder in the dielectric response of BaTaO2N,
 B. Ravel, Y-I. Kim, P.M. Woodward, and C.M. Fang, Physical Review B,
 73, p. 184121 (2006)

I used a DFT result as the basis of my fitting model in that paper.
The naive approach was to make a Feff path from every absorber/
scatterer pair in the box.  That's certainly a good idea, but Ifeffit
has some compiled-in limitations that made that impractical.  My
solution was to make an evenly spaced grid in R and bin together paths
with similar path lengths.  That reduced the problem to a tractable
number of Feff paths at the expense of writing a one-off program as a
processing step between the DFT and setting up the EXAFS fit.

If my answer is relevant to your problem, I have a few other ideas
about how to approach the problem that we can discuss futher, if
you'd like.

B

-- 
 Bruce Ravel  ---------------------------------------------- bravel at anl.gov

 Molecular Environmental Science Group, Building 203, Room E-165
 MRCAT, Sector 10, Advanced Photon Source, Building 433, Room B007

 Argonne National Laboratory         phone and voice mail: (1) 630 252 5033
 Argonne IL 60439, USA                                fax: (1) 630 252 9793

 My homepage:    http://cars9.uchicago.edu/~ravel 
 EXAFS software: http://cars9.uchicago.edu/~ravel/software/