[Ifeffit] a basic (?) EXAFS question
Scott Calvin
SCalvin at slc.edu
Mon Apr 18 08:31:02 CDT 2005
Hi Juro,
Just a note of caution--if I've followed this discussion, you're
fitting the amplitude for each shell separately. While there are
sometimes good reasons to do this (it's the way Anatoly Frenkel
determines crystallite size and morphology, for example), if you
think you have fairly ordered, fairly large (> 10 nm, say)
crystallites, then you need a physical justification. Although the
match between fit and data will of course usually look marginally
worse by using the same amplitude for every shell, it often has the
side benefit of reducing uncertainties in the guessed parameters.
In terms of fitting strategy, when I was an EXAFS neophyte I would
start my fits by having lots and lots of parameters and then would
gradually reduce the number. Since then I've switched around to what
most people recommend, which is starting with very few parameters
(many recommend starting with none!), and then adding guessed
parameters as necessary. The latter has the benefit of forcing you to
think about what's physically reasonable, reducing the incidence of
bizarre "false minima," giving you a sense of what's going on early
in the process, and being easier to defend in print.
--Scott Calvin
Sarah Lawrence College
>Matt,
>
>thanks a lot for the few simple questions. One of them -
>the R range - seems to be the problem, since the range
>wasn't big enough. Oh well ... The structure of loellingite
>has very well defined shells, up to the fifth one.
>
>Thanks again,
>
>Juro
>
> > On Sun, 17 Apr 2005, Juraj Majzlan wrote:
> >
> >
> > > I have a question about fitting an EXAFS dataset. The
>> > compound is FeAs2, crystalline, and will be used as a
>> model
>> > for further study. I was able to fit the first three
>> shells
>> > very well. However, when I try to fit the fourth shell,
>> I
>> > either get amplitude that is too high (> 2, with large
>> > errors), or 0.0. There are several single-scattering
>> and
>> > several multiple-scattering paths in that region.
>> > Sometimes, only excluding one of the MS paths leads
>> > immediately from amplitude of >2 to 0.0.
> >
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