[Ifeffit] FEFF8 vrs FEFF6

[Matt Newville]

HI Shelly,

I wouldn't say it's wrong to use Feff8 for EXAFS analysis, just rarely
necessary.

Feff8 was definitely designed for XANES, and improvements for EXAFS
were not the main goal.   But there are a few cases where Feff8 _is_
known to be better for EXAFS than Feff6.  (I'll leave out Feff7 as it
seems no one is using this much).  Clusters with H is one, and
transuranic elements is another.  I've heard a few people claim that
Feff8 gives "better E0's", which I think means that  E0 refines to a
value closer to 0 (that is, to where the it's arbitrarily assigned in
the data processing).  Since E0 has to be refined and has little
physical significance, I don't find this to be a compelling advantage
for Feff8.

Early versions of Feff8 (pre-release?  Including 8.00?? I'm not sure)
were much worse for EXAFS.This did get fixed,  but I'm not sure when
(I have a hard time understanding Feff8 version numbers and can never
really tell which get released and widely used: I've given up
complaining and live wih the fact that "Feff8" means one snapshot of 8
years worth of work.).   By Feff8.1 for sure, and maybe eralier, there
were essentially no difference between Feff6, 7, and 8 for EXAFS.  
That was based on analysis of data on metal foils (Cu, Ag, Au), and
could hardly be called thorough and possibly not even 'highly relevent
to real-world problems', as it doesn't even test changes in exchange
or overlap of metal-oxygen potentials.   Better testing and
comparisons would be very good.

Anyway, I think it's fine to use Feff6, 7, or 8 for EXAFS.

I don't think anything should change for these versions in the
reporting of how a Feff calculation was done for EXAFS.   Most EXAFS
work using Feff give no details about the calculation, and I think
that's mostly OK, as the Feff/Atoms/Artemis default parameters are
generally quite reasonable.   If the work discusses comparing
different Feff calculations, then obviously more detail on what was
changing would be needed.

That's my opinion, anyway.

--Matt