[Ifeffit] Using radius in FEFF to determine nanoparticles

[Bruce Ravel]

On 09/11/2013 10:50 AM, Lisa Bovenkamp wrote:

Hi Lisa,

I think Anatoly basically covered your questions, but I wanted to make
a few more comments.

> a) a small SCF radius the feff code might not converge (which cannot
> give good results),

I am always a bit troubled by comments like this.  Whether or not a
calculation has converged in terms of a parameter like the SCF radius
is not something you need to speculate on or even ask an "expert"
about.  You can just go test it yourself.  Try computing things with
several increasing values of radius and see what changes.  People
publish questionable things all the time.  (Just read any of my
papers!)  Happily, we do not have to take things on faith.

> b) when the FMS radius is small - and there are not many scatterers
> included - the resolution of the calculated XANES spectrum is "bad"
> and features are broad.  I think that has nothing to do with the
> nanoparticles

I would use somewhat more specific language here.  "Bad" is a value
judgment.  I think you meant to say that the small FMS radius results
in features being missed in the calculation due to scattering from
more distant atoms.

It is important to test convergence of both the SCF and FMS radii.
They usually (perhaps always) are different.

> c) a correct real space feff table of xyz coordinates should include
> more than just one nanoparticle since the neighboring particles do
> have an effect on each other - or am I wrong?

It is important to remember what we are measuring in an XAS
experiment.  We only see signal when there is a local correlation
between the positions of the absorber and the surrounding
scatterer(s).  In general, this means that there has to be bond
between the atoms or they must be within some kind of network of
bonds.

Two adjacent nanoparticles almost certainly cannot have that sort of
correlation because the atoms that make up the two nanoparticles are
not within the same network of bonds.

To put that another way, any pair of adjacent nanoparticles will have
some orientation one with respect to the other.  But any other pair of
nanoparticles almost certainly will not have the same orientational
relationship.  XAS averages over a large ensemble of such particles.
Unless something very special is happening, the universe -- as seen by
the absorber -- ends at the boundary of the nanoparticle.

HTH
B



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

  National Institute of Standards and Technology
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