Using radius in FEFF to determine nanoparticles
Hey all, I started working on modelling nanoparticles with FEFF. 1. Anybody have some useful links about this subject? 2. In 2 reports people were using the radius (i.e. number of atoms used in the calculations) as a measure for the size of the nanoparticles. Further they were discussing the absence of features due to the size. This strikes me odd. I thought that for a) a small SCF radius the feff code might not converge (which cannot give good results), 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 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? Thanks. Lisa
Lisa,
The number of papers using FEFF to model nanoparticles is in the thousands.
Starting with FEFF8, most XANES features can be reliably modeled.
Look for papers by J.Kas and others from J.Rehr group for details.
Neighboring nanoparticles do not contribute, one reason being that XAS is dominated by the interior of the particle, and there are many other reasons, but that one is the most important.
Too few atoms on the first particle will see the atoms on the second particle to make contribution to average XAS. These contributions will be also strongly disordered and tend to cancel each other.
Anatoly
Sent from my iPad
On Sep 11, 2013, at 10:57 AM, "Lisa Bovenkamp"
Hey all,
I started working on modelling nanoparticles with FEFF. 1. Anybody have some useful links about this subject?
2. In 2 reports people were using the radius (i.e. number of atoms used in the calculations) as a measure for the size of the nanoparticles. Further they were discussing the absence of features due to the size. This strikes me odd. I thought that for a) a small SCF radius the feff code might not converge (which cannot give good results), 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 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?
Thanks. Lisa
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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@bnl.gov National Institute of Standards and Technology Synchrotron Science Group at NSLS --- Beamlines U7A, X24A, X23A2 Building 535A Upton NY, 11973 Homepage: http://xafs.org/BruceRavel Software: https://github.com/bruceravel
participants (3)
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Anatoly I Frenkel -
Bruce Ravel -
Lisa Bovenkamp