Hi Juan,
The same S02 should be OK; that's generally thought to be chemically
transferable.
Unless I'm misunderstanding something about your system, there is no
physical basis to justify the same delr. Think about it: why would
both phases show the same difference in bond length between the model
and the actual structure?
Using the same sigma2 is also problematic; it seems unlikely that the
disorder and vibrational spectrum is the same in the two cases.
deltaE0, to first approximation, may be the same. In both phases the
valence of the absorbing atom is the same (i.e. 0), so assuming the
same E0 is not unreasonable.
To my mind, therefore, you need at least 6 parameters (plus whatever
else you're using; e.g. the fraction of each phase present).
So I'll use this opportunity to beat one of my favorite drums: why
not try a multishell fit? It sounds to me like you have phases that
are likely to be fairly crystalline. If your data is decent, you may
be able to extend your fit out to several coordination shells without
adding many new free parameters. (You have to do this in a physically
reasonable way: delr might need to be scaled by reff, etc.) This
might be a net gain in degrees of freedom, and also might reduce
uncertainties in some of the fitted variables by reducing correlations.
--Scott Calvin
Sarah Lawrence College
At 03:25 AM 11/9/2006, you wrote:
Hi people!!
I was fitting with Artemis bimetallic catalysts with two different
structures:
platinum structure plus an intermetallic compound structure. As I only use a
small range of R space (1st shell) for fitting, I have a small number of
independents points and I used an appropriate number of variables. The
question is: I used the same values of SO2, delR, sigma^2 and
deltaE0 for both
structures, is that ok? or maybe I have to increase until 8 the number of
variables?
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