John, On Tue, 4 Mar 2003, John J. Rehr wrote:
Doing XANES fits ala Benfatto and Della Longa would also be desirable in the long run, i.e., fitting the XAS(x) with x standing for various parameters like interatomic distances. To do that would generally require looping over the FMS module in FEFF and could get time consuming. To avoid this, one needs something like a tensor XANES calculation, which computes d mu/ dx via multiple XAS calculations and hence permits fast linear interpolations. This is not implementable with IFEFFIT/ATHENA at present as far as I know. However, I don't see major difficulties in implementation (?).
Well, it wouldn't be easy, but it is technically feasible. Bruce and I have been pushing for changes to Feff that would facilitate these and other important capabilities for many years now. The main difficulty is a legal one. The Feff license forbids anyone from altering Feff8 and distributing the changes. So I agree when you say that these changes cannot be implemented within IFEFFIT. This is not for any technical reason, but simply because you do not allow it to happen. You are unwilling to change the code to allow these things, and forbd anyone else from making such changes. This is not an unfortunate but unavoidable consequence of an otherwise beneficial license. No, it is the entire point of the Feff license to restrict others from developing new capabilities exactly of this sort. There is no other explanation for the license. The scientific merit of this policy is beyond my comprehension, but the restriction is completely deliberate. Perhaps the more surprising thing is that Feff has never included analytic capabilities, but is widely used for EXAFS analysis. In this respect, Feff is very unusual for scientific software. Since those working on analysis codes are forbidden from changing Feff, it's not very likely that significant progress will be made in XANES analysis or other new areas unless you pay attention to the requests and suggestions of experimentalists and authors of analysis codes. History is does not on your side. And yet from the above statement, you see these capabilities for XANES analysis as technically possible and desirable. I agree with this, but don't see how they can possibly be done given the very clear intention of the Feff license. I cannot imagine anyone willing and able to make these changes. Since you think these capabilities for XANES analysis are both desirable and possible, it would be interesting to hear why you think it is in the best interest of the XAFS community for the Feff Project to prevent the development of such capabilities. --Matt