[Ifeffit] Other programs than FEFF - GNXAS and MXAN

[Matthew Marcus]

>> 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 
specifying
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.
    mam