[Ifeffit] Artemis FEFF for clusters >500 atoms

[Anatoly Frenkel]

Re: [Ifeffit] Artemis FEFF for clusters >500 atomsIn 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 at millenia.cars.aps.anl.gov
[mailto:ifeffit-bounces at millenia.cars.aps.anl.gov]On Behalf Of
scalvin at 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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