[Ifeffit] Other programs than FEFF - GNXAS and MXAN

Matthew Marcus mamarcus at lbl.gov
Sat Jul 29 16:06:59 CDT 2006


>
> Feff deliberately did not include graphics or analysis, with the hope /
> expectation that lots of other people would do this. This turned out to
> be true, and very many analysis programs use Feff.   It is perhaps
> interesting that this has not happened for GNXAS.  In contrast, it is
> uninteresting for why this did not happen for EXCURVE: they gave
> an exclusive license to a company, so that regardless of how good
> that theory is, they chose for it to never be in wide use.

Including only bare-bones ASCII in/out is, IMHO, the only way to obtain true 
portability,
and it has worked.  I vaguely recall that they use some sort of verifier on 
their code to screen
out system dependencies.

>> Bruce discussed this, and I may get some of the details wrong (hopefully
> someone will correct me) but:   GNXAS models g(R) directly and applies
> this in an integral of the EXAFS equation.  It does not use a sum over
> paths with a set of cumulants for each path.  I do  not know what
> limitations are put on g(R), but I think they may have a few lineshapes
> used to model g(R) (I do have the documentation somewhere!).    If I
> remember correctly, GNXAS also fits the full mu(E) spectra,  including
> the background and EXAFS, but this is not what I consider "interesting"
> about GNXAS.

If it uses a g(r) approach, then I imagine that it doesn't handle multiple 
scattering.  On the
other hand, it could be argued that *in the EXAFS range* a system which 
needs a g(r) approach
won't have detectable MS.  That still leaves XANES, though.

My shell-by-shell EXAFS fit program has the option to use as g(r) a 
Gaussian-broadened exponential
tail function, whose FT is analytically simple and which captures the 
essential features of a somewhat-disordered
shell.  I prefer it over cumulant expansions because it doesn't assume that 
C6 and up are 0 and it gives you
a physically reasonable g(r) with a minimum of parameters.

I gather that GNXAS and MXAN are hard to use, with many, many non-obvious 
parameters and
(MXAN) no interface to speak of.  Is this the sort of thing where one must 
spend months learning,
and are likely to come up with plausible but incorrect solutions in the 
meantime?  I'd hate to publish
something that was wrong because I didn't set a switch correctly in a 
program.

>
> That's not about "theory" as analysis. I don't know what theoretical
> advantages GNXAS might have.  I believe it includes curved wave effects,
> and that it uses the Hedin-Lunqvist exchange (perhaps the only option?).
> I think that it has a simple polarization model  (perhaps only dipole??). 
> I
> believe it does multiple-scattering only out to four-leg paths  (good 
> enough
> for almost  all cases), and don't know how it does this.  I recall from 
> one of
> the GNXAS papers that they talked about correlating Debye-Waller factors
> for MS paths with those of SS paths, but I don't know that works in 
> practice.

All that sounds like what FEFF has.

>
> Sure, though I'm not sure that MXAN has been demonstrated to be able
> to do this better than other XANES approaches.  I may have missed some
> papers, but I've mostly seen it applied to small molecules.

Yes, it's for smallish molecules.  Typical center-scatterer distances are 
<4-5A in the examples I've seen.
What 'other approaches' to XANES are publically available which work better 
than FEFF8.1?

>
> I got a copy in 2001 by asking Benfatto (Maurizio.Benfatto at lnf.infn.it).

I got an email showing that this is still the path.  I just discovered that 
this email came from this mailing list (oops - should have
read the header), so my reply will be on everyone's screen.  Sorry about 
that!
    mam 




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