In the articles by Montejano-Carrizale et al, Nanosctructured Materials, v.1, 397 (1992) and v.8, 269 (1997), analytical expressions are published for the 1NN coordination numbers in different geometries, including cuboctahedral, as a function of the cluster order L. However, to fit the entire r-range, one has to know how the 2NN, 3NN and multiple-scattering paths depend on the cluster order. Very similar to what Scott described, we also calculated these truncation parameters for SS and MS paths, for any given cluster order L, but they were not parameterized during the fit since it requires to know analytical expression N(L) where N is the truncation parameter and L is the cluster order. Instead, we just wrote several programs - coordination number generators - for different geometries: cuboctahedral, icosahedral, octahedral, truncated octahedral, and for each geometry and for each size we obtained the truncation parameters for parsing the x,y,z coordinates through another program that we wrote, that calculates the radial distribution function relative to the central atom. By EXAFS data analysis we obtained the coordination numbers for SS and MS paths model-independently and then compared them against several models of different sizes (L) and morphologies. Here are some references:  J. Am. Chem. Soc., 119 , 7760 (1997) ,  J. Synchrotron Rad., 6 , 293 (1999).  J. Phys. Chem. B, 105, 12689-12703 (2001).  
 
Anatoly
 
 
 
-----Original Message-----
From: ifeffit-bounces@millenia.cars.aps.anl.gov [mailto:ifeffit-bounces@millenia.cars.aps.anl.gov]On Behalf Of scalvin@slc.edu
Sent: Monday, May 30, 2005 9:08 AM
To: XAFS Analysis using Ifeffit
Subject: Re: [Ifeffit] Artemis FEFF for clusters >500 atoms

Hi Charles,

We didn't use FEFF to create the really large clusters. Instead, we just
generated a FEFF cluster larger than the largest paths we fit, and then
used constraints to deal with all the termination effects. For the
detailed modeling of cuboctahedra (it's been a while--that was the
expected morphology, right?), we reduced the amplitude of each path by the
weighted average of the reduction in coordination number relative to the
bulk for the given morphology and size. A very clever high school student
I was working with (C. J. Riedel) automated the process of generating
these constraints.

This approach is marginally less accurate than letting FEFF figure out an
entire cluster, but doesn't require nearly as much computer power, and
should be pretty darn close for large clusters.

--Scott Calvin
Sarah Lawrence College

> Hi Scott,
> Thanks for your suggestion.  I find that I need to generate FEFF files for
> larger clusters than I had initially started off with. In your recent
> Physica Scripta paper [T115 (2005) 744-748], you had looked at some pretty
> large Pd clusters, particles 0.1 to 200nm in diam. (page 745, col 2, para
> 2, lines 5-6).  Some of these clusters are well over 500 atoms. How did
> you
> manage to create the FEFF?  When I try to do so for a cluster this large
> in
> Artemis, the program flags an error saying that I have too many atoms (in
> my *.inp file generated from TkAtoms).
> Charles
>
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