[Ifeffit] Other programs than FEFF - GNXAS and MXAN

Matthew Marcus mamarcus at lbl.gov
Sun Jul 30 09:29:08 CDT 2006

>> 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.
> I don't understand this.

A verifier is a program that inspects programs, looking for probable bugs 
such as un-initialized
variables, non-standard syntax, and system dependencies.  In fortran and C, 
graphics is
not natively provided, so much be called from a library.  Such libraries 
vary from system to system.

>> 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.
> Actually, GNXAS does handle MS.  It's not clear to me how well it handles
> small crystallographic distortions, mixed shells, and so on, but it does
> include MS.   I agree with you that requiring one of g(R) and MS seems
> to suggest that the other isn't necessary.

It may use MS for systems in which the atomic positions are defined, but 
g(r) alone
does not define them.  To get both *at one time* would require some way of 
baseline atoms positions plus some random distortions with specified 
distributions, plus
models for how those distortions are correlated.  Essentially, calculating 
MS requires assuming
things about 3-body correlations at least.
>> > 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.
> Err, no.  Feff has multiple options for exchange potentials, and decent
> models for loss terms.  Feff has polarization dependence beyond dipole,
> which is needed for L edges, and can do elliptical polarization fot XMCD.
> I think GNXAS has none of that.

I meant that FEFF's options are a superset, not that they are identical.

> For DW factors for MS paths, Feff can use the correlated Debye model.
> GNXAS claims to do something different and more complicated, but I've
> never understand this, or why that would be important for systems that
> needed to be treated with a g(R).

If GNXAS provides two different systems, one for disordered systems 
described by g(r)
and one for systems described by atomic positions, then maybe the 
'different&more complicated'
part is only for the atom-position case.

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