[Ifeffit] Question about FEFF6

[Bruce Ravel]

Jack,

You have rediscovered the conventional wisdom about properly bounding
a cluster for calculation with Feff.  When there are not enough atoms
on the periphery of the cluster to constrain the size of the muffin
tins, the muffin tins tend to be much too large.  As you can see this
has consequence for the scattering function calculated by Feff.

In your case, the muffin tins of the oxygen atoms are about 0.73
Angstroms when including even a single P atom on the periphery of the
cluster.  For the first calculation Rmt is 1.06 -- substantially
larger.

What's interesting is that a single P atom is sufficient to make the
calculation of the O contribution work out reasonably well.

Have you read Shelly's book chapter review of EXAFS?  In her example
on determining a second shell atom type, she runs Feff calculations on
known crystal structures with various second shell atoms.  She then
takes just the kinds of paths she needs to model different
contributions to the EXAFS.  The reason she adopted this strategy
rather than trying a small, notional cluster of the sort the you have
made in your example is because constraining the muffin tins is quite
important.  Indeed, the potentially counter-intuitive strategy of
snarfing paths from some related crystal structure works well
precisely because it is a way of guaranteeing sensible muffin tin
radii.

HTH,
B

P.S. You can see the muffin tin radii by opening the .apj file as a
zip file (which is what it is!) and looking at the misc.dat file in
the dataX.feffY folders.


On Monday, April 11, 2011 02:33:54 pm goodhei8 at gmail.com wrote:
> Dear IFEFFIT Members,
> 
> 
> I have a strange problem with FEFF calculation. I am not sure if I made
> some mistakes in FEFF of input file, or FEFF did not handle it well.
> 
> For Oxygen paths calculated with FEFF in Artemis by 3 input files, the
> results for O are supposed to be close, but I got a big difference, when
> I transform them into r space(k 2.5~10,kweight=2 in attachment).
> 
> There are three input files for FEFF:
> #1, 6O only on xyz axis
> #2, + 1P besides 6O
> #3, + 6P besides 6O
> 
> For O, #2 and #3 are close(almost same), but #1 is much bigger. (It
> would make sense to me if #1 #2 are close.)
> 
> Thank a lot.
> 
> Best,
> 
> Jack Song

-- 

 Bruce Ravel  ------------------------------------ bravel at bnl.gov

 National Institute of Standards and Technology
 Synchrotron Methods Group at NSLS --- Beamlines U7A, X24A, X23A2
 Building 535A
 Upton NY, 11973

 My homepage:    http://xafs.org/BruceRavel
 EXAFS software: http://cars9.uchicago.edu/~ravel/software/exafs/