[Ifeffit] Other programs than FEFF - GNXAS and MXAN

Federico Boscherini federico.boscherini at unibo.it
Sat Jul 29 17:24:19 CDT 2006

Hello everyone,
there seems to be an interesting exchange of ideas going on. Let me contribute a few ideas or bits of information:
1) I don't believe anybody holds the right to speak in the name of the "international community", whatever that means. The scientific process is rather complex and does not even necessarily adapt itself to rules of democratic representation (who votes?...). Obviously the peer review process is not ideal (see the referee with strange ideas about a particular version of feff) but it might be the best we have.
2) Merci, Francois, for reminding us about the recent history of XAS. At the recent XAFS 13 conference there was hardly any mention of AXAFS and multielectron excitations. Is there anyone willing to write a history of XAS to follow on the one by Ch. Brodeur? Might be an interesting endeavor.
3) I have been using both GNXAS and FEFF related software for many (say 15 - 20) years. Most of the differences have come up in replies in the past few days. One extra difference between GNXAS and FEFF I would like to point out is that FEFF treats (linear and circular) polarization dependence while as far as I know GNXAS does not (it didn't for sure until a few years ago).
4) The formalism underlying GNXAS is very elegant and reading the original papers is great. I suggest it is highly worth the effort.
5) Regarding ease of use: is XAS amenable to a "black box" approach? Despite the fact that this clashes with what many of us teach to physics students, it might be very useful e.g. in the field of bioXAS. But can this be done? Or is the underlying physics too complex? 
Prof. Federico Boscherini
Department of Physics
University of  Bologna
viale C. Berti Pichat 6/2
40127 Bologna (Italy)
e-mail: federico.boscherini at unibo.it
tel: ++39 051 209 5805
fax: ++ 39 051 209 5153
research web site: http://www.bo.infm.it/grp/fb/e2_home.htm
sito della didattica: http://ishtar.df.unibo.it  (seleziona Uni Bo, Scienze, Boscherini)


From: ifeffit-bounces at millenia.cars.aps.anl.gov on behalf of Bruce Ravel
Sent: Fri 28/07/2006 16:03
To: XAFS Analysis using Ifeffit
Subject: Re: [Ifeffit] Other programs than FEFF - GNXAS and MXAN

On Thursday 27 July 2006 14:22, Matthew Marcus wrote:
> Other reasons for the popularity of FEFF include its longer history,

Actually the early references for both GNXAS and FEFF6 date from 1991.

To paraphrase Steve Zabinsky (the fellow who did much of the work on Feff6)
"if you make it easy to use, everyone will use it -- even if it's not

> its distribution as source, so 
> it can be recompiled for any system, and not least, IFEFFIT/Artemis!

I'd say that today this mailing list contributes to Feff's wide adoption at
least as much as the codes that are the topic of the list.

> What is this 'interesting approach'?

Matt and I were solving materials physics problems in grad school, so our
codes are very well suited to problems involving crystals or other things
with fairly well known atomic positions.  Adriano and Andrea were (I think,
but I could be mistaken) solving problems more along the lines of solvation
chemistry and highly disordered material.  Consequently (and explained in
broad brushstrokes) GNXAS is well suited to problems for which the g(R) is
not described by discrete atomic positions.  In essence, GNXAS integrate the
exafs equation over a functional form of g(R).  It is certainly possible to
deal with high disorder in Feff-base analysis and it is certainly possible to
do material science-y problems with GNXAS -- they really aren't that
fundamentally different.

If you are looking at amorphous solids or solvated ions (or really any other
system) you would be wise to look into GNXAS.  You may find that it fits your
problem or fits into your brain better than Feff, Ifeffit, and my software.

> That reminds me - I'm a co-author on a paper, which was submitted to PRL,
> in which we used FEFF8.1
> to simulate EXAFS.  A referee complained that FEFF8.1 was for XANES, not
> EXAFS.  What was that
> about?

Ummm.... a high degree of confusion on the part of the referee perhaps?


 Bruce Ravel  ---------------------------------------------- bravel at anl.gov

 Molecular Environmental Science Group, Building 203, Room E-165
 MRCAT, Sector 10, Advanced Photon Source, Building 433, Room B007

 Argonne National Laboratory         phone and voice mail: (1) 630 252 5033
 Argonne IL 60439, USA                                fax: (1) 630 252 9793

 My homepage:    http://cars9.uchicago.edu/~ravel
 EXAFS software: http://cars9.uchicago.edu/~ravel/software/
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