Hi EXAFS friends, I was reading a paper this morning where the authors used a small hydrated cluster in Feff8.2. The cluster has a metal center surrounded by one shell of oxygen atoms. All feff paths in the calculation were used in the model fits. No mention was made in the manuscript of the muffin tin radii used in the Feff8.2 calculation based on this small cluster, hence I assume that these authors were unaware of the potential pit falls in doing so. Using feff7, I have run a comparison of the calculated signal from a bounded and unbounded shell of atoms at the same distance. I found that the unbounded shell of atoms has a muffin tin radii that is really big because it is not bounded by any other atoms giving rise to an error in the calculated signal from these atoms. Has anyone run this experiment with Feff8.2? I have always used the metal-oxide crystal structure in my modeling of hydrated metal ions and have found reasonable agreement with chemical speciation calculations. I believe that the non-expert users of Feff, find it difficult to remember not to use the exact model within Feff because of this "feature". Is there any discussion of adding a hydration sphere option into Feff? I look forward to your comments. Shelly Shelly Kelly Bldg 203 RM E113 Skelly@anl.gov Argonne National Laboratory 630-252-7376 9700 S Cass Ave www.mesg.anl.gov Argonne, IL 60440
On Thursday 06 October 2005 10:21, Kelly, Shelly D. wrote:
All feff paths in the calculation were used in the model fits. No mention was made in the manuscript of the muffin tin radii used in the Feff8.2 calculation based on this small cluster, hence I assume that these authors were unaware of the potential pit falls in doing so.
Using feff7, I have run a comparison of the calculated signal from a bounded and unbounded shell of atoms at the same distance. I found that the unbounded shell of atoms has a muffin tin radii that is really big because it is not bounded by any other atoms giving rise to an error in the calculated signal from these atoms. Has anyone run this experiment with Feff8.2?
I think that Feff8 will have the same problem with muffin tin size in the case of a small unbounded cluster as Feff6 always has. The effect might be mitigated somewhat by self-consistency. Or not. Knowing that would require testing that I certanly have not done myself. In any case, I would agree with you that it is probably better practice to use a cluster in the feff input file that extends at least one shell beyond the furthest shell you intend to analyze.
I believe that the non-expert users of Feff, find it difficult to remember not to use the exact model within Feff because of this "feature". Is there any discussion of adding a hydration sphere option into Feff?
I discussed this several times with John back in grad school, but don't know if anyting formal has been done in Feff8 with respect to an outer sphere -- but I might just be ignorant. That's certainly been known to happen ;-) Perhaps John can shed some more light on this.... B -- Bruce Ravel ---------------------------------------------- bravel@anl.gov Molecular Environmental Science Group, Building 203, Room E-165 MRCAT, Sector 10, Advance Photon Source, Building 433, Room B007 Argonne National Laboratory phone and voice mail: (1) 630 252 5033 Argonne IL 60439, USA fax: (1) 630 252 9793 My homepage: http://cars9.uchicago.edu/~ravel EXAFS software: http://cars9.uchicago.edu/~ravel/software/exafs/
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Kelly, Shelly D.