[Ifeffit] Generating single scattering paths in Artemis

[scalvin at slc.edu]

Hi all,

Thanks, Mike, I'd never tried that option before. But I agree with Carlo
that it's not that easy a way out, since you still have to understand how
to assign potentials in a FEFF.INP file...exactly the sort of thing the
Artemis front-end is supposed to protect newbies from, I think.

A question for Carlo--I guess I'm not clear on what the impact is of
tetrahedral vs. octahedral or whatever if all you're doing is a
single-scattering nearest-neighbor EXAFS analysis. Isn't a symmetric
octahedral arrangment just the single-scattering path with a degeneracy of
6; a tetrahedral a degeneracy of 4; etc.? Why have FEFF calculate anything
other than the single-scattering path?

Having said that, I could see an option that lets you generate a single
FEFF path via the Artemis front end (and maybe even specify a degeneracy
up front). That way you wouldn't have to deal with any of the rigamarole
of choosing a low-symmetry space group and putting your atoms somewhere
appropriate in the unit cell. It would certainly make things easier for me
on occasion (sometimes I have an amorphous component mixed in with the
crystalline stuff, for example).

The one thing that makes me nervous about this is that I almost don't want
to make single-shell single-scattering fits =too= easy for novice users. I
work with a lot of undergraduates, and in my experience it's hard for
people new to EXAFS and characterization in general to understand just how
unreliable such fits can occasionally be, due to high correlations between
fitted parameters, leakage from higher-R shells, issues with the
background, etc.. A fit with more shells and/or multiple scattering is
more likely to =obviously= fail (e.g. yield a poor match between fit and
data) if something is wrong. So I guess I kind of like there to be a
little bit of a learning process new people have to go through before they
can make Artemis do anything, so that they least understand that there can
be paths beyond the first. :) But this is really just a gut feeling--it's
possible that a single path shortcut like Carlo describes will just make
the learning curve easier...

--Scott Calvin
Sarah Lawrence College