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@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@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/