[Ifeffit] Other programs than FEFF - GNXAS and MXAN
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
>> 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,
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
>> 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
does not define them. To get both *at one time* would require some way of
baseline atoms positions plus some random distortions with specified
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
part is only for the atom-position case.
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