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?