[Ifeffit] XANES

[Matt Newville]

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