On Thursday 27 May 2004 02:18 pm, Wojciech Gawelda wrote: WG> My question is the following: does anyone of you have some experience WG> with such procedure? And if yes, shall than distinguish between the WG> first shell of nearest neighbors and the rest of the atoms in terms of WG> their E0 corrections (using 2 parameters)? Or perhaps one can use WG> separate E0's for each path? Hi Wojciech, Feff makes approximations when it computes the potentials. The farther away from a muffin tin the real material is, the worse those approximations will be. For a material that is centro-symmetric or not so far away, the potentials should be pretty trustworthy in the extended spectrum. For an assymmetric material, the muffin tin approximation may be suspect. In particular, there is no real concept of a bond in a muffin tin potential. In a real material, there might be phase shifts seen by the photoelectron as it scatters in one direction that are different in another direction. One empirical way of accommodating the shortcomings of a muffin tin potential is to introduce more than one e0 into a fit. But doing so is like approaching the part of an ancient map that says "Here there be dragons." The dragon in this case is the argument that you are just throwing a non-physcial parameter at the problem in order to obtain a fit that is numerically superior. In slangy English, we call these fudge factors. Fudge factors may be bad because they may be highly correlated with the parameters we want to measure. A fudge factor that improves the fit but which cannot be ascribed any physical interpretation should be viewed with suspicion. You should note that the first e0 is NOT a fudge factor. As I mentioned in response to Stefano's mail yesterday, the first e0 is needed to make sure the absolute energy scales of the data and the calculation line up. Thus the first e0 does have a physical interpretation. The bottom line for considering a second e0 is that it must be physically justifiable. If it no more than a fudge factor, it's probably best to stay away. I would insist that all the parameters affecting the various delta_R values all behave sensibly as I change some extrinsic parameter. By this I mean that if I measure, say, a temperature sequence, all the delta_R parameters show a dependence on temperature that is sensible and consistent with anything I might know about the behavior of the material. Temperature is a good way of putting a prior constraint on parameters. There are other ways as well. Similarly sigma^2 values should be sensible, as should any other parameters. I would not expect the best fit values to be hugely different, perhaps not even outside their error bars, when I add the second e0. If adding the second e0 improves the fit quality and tightens other uncertainties without significantly changing best fit values, then I might begin to consider it -- especially if there is some correlation between the paths that need a different e0 and a suspected shortcoming of the muffin tin approximation. Another case where a second e0 might be justifiable is if you expect there to be interstitial electron density in a particular direction (due to strong bonding that the muffin tin approximation knows nothing of) and the use of multiple e0's in particular paths correlates very strongly with what you expect from the real electron density distribution. Again, things that can be justified physically can be considered in the analysis. Actually the argument that parameters should be physically justifiable applies to all parameters, not just your e0 parameters ;-) HTH, B -- Bruce Ravel ----------------------------------- ravel@phys.washington.edu Code 6134, Building 3, Room 405 Naval Research Laboratory phone: (1) 202 767 2268 Washington DC 20375, USA fax: (1) 202 767 4642 NRL Synchrotron Radiation Consortium (NRL-SRC) Beamlines X11a, X11b, X23b National Synchrotron Light Source Brookhaven National Laboratory, Upton, NY 11973 My homepage: http://feff.phys.washington.edu/~ravel EXAFS software: http://feff.phys.washington.edu/~ravel/software/exafs/