[Ifeffit] FEFF8 vrs FEFF6

John J. Rehr jjr at leonardo.phys.washington.edu
Mon Feb 6 01:05:30 CST 2006


Hi Everyone,

   The newest release of FEFF8 is version 8.20 (according to the FEFF web site 
http://leonardo.phys.washington.edu/feff ). This version is also the most
stable and fixes bugs in previous releases (8.10 , 8.00 and all experimental
versions, which are designated with an x e.g., 8x39).  In my view all 
earlier versions than 8.20 should be trashed. Matt is right that we haven't
been consistent with version nomenclature, since 8.20 or 8.2 are often
used interchangeably.

    Matt summarized well the differences between FEFF6 and FEFF8 in a
previous message to this list. In my view, the key difference in FEFF8
for EXAFS is the availability of self-consistent scattering potentials
and hence a better approximation of E0. In my opinion reducing the
uncertainty in E0 should improve EXAFS analysis since this should also
reduce uncertainties in distance determinations. However, the differences
may not be that significant. Thus for EXAFS FEFF6 is often just as good.
On the other hand, FEFF8 also yields electronic structure, e.g., charge
counts, charge transfer, and densities of states, information which
complements the structural information from EXAFS alone.

    A drawback in using FEFF8 is that getting the SCF loop to converge for
complex materials can be tricky, and one of our challenges has been to
develop a more automated procedure. Until that is available, we recommend
always using the recommended defaults in FEFF8.2 in the SCF potential 
construction, unless one has good reasons for doing otherwise.

   We would be very interested to learn of cases where FEFF8.2 yields
results which are inferior to those from FEFF6.

   J. Rehr



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