Dear fellow EXAFSeteers,

I have a somewhat bizarre problem with Artemis fits. I am using Demeter 0.9.25 
64bit on a Windows7 machine.

The Situation: I am trying to fit the first shell complexing ligands to aqueous 
metal complexes. I have sets of experiments at different temperatures but 
otherwise identical conditions and concentrations. When I fit all experiments 
separately, parameters such as coordination number and metal-ligand distance 
show systematic variation with temperature (as they should). Therefore, I would 
like to try fitting all these experiments together and define these parameters 
as functions of temperature, which would reduce the total number of free 
parameters relative to the experimental points and should also bring the 
uncertainties down a bit. To this end, I first made an Artemis project that 
contains several of the data sets (at different temperatures). Then I made 
individual parameters for each data set that are exactly as they are in the 
individual fits (parameter names are different, though) and duplicated the 
fourier transform parameters of the individual fits. Now each data set has its 
own set of parameters. No parameters are shared between data sets and there are 
no connections between data sets via "def" functions in the GDS or anything.

The Problem: When  start the fit, I get different results from the individual 
fits. Still mostly within the calculated error values (which are quite large at 
the moment), so not "significantly different" in the statistical sense of the 
words but still very different numbers. Whats more, if I add another data set 
with corresponding parameters (again no connection to the other data sets or 
parameters), the results for the parameters of the other data sets change again.

Am I right in assuming that this should not be the case or is there a 
connection which I am missing? Might this be a bug? Some fits react a bit 
sensitive to the choice of k- and R-space windows (but these were identical in 
individual and simultaneous fits). My fits are pushing the number of 
independent parameters a bit, according to the log-files (which is why I want 
to do the simultaneous fits with T-dependent parameters). Might this cause 
instability in the fit routine. In the simultaneous fits, the number of highly 
correlated parameters (>0.95) skyrockets (is ok in individual fits), which may 
be related to the next point. I suspect it may also have to do with the way 
Artemis decides that the fit is "done". If this is evaluated globally for all 
parameters somehow, then the observed behavior would not be surprising. But if 
it is evaluated on a per-parameter or per-data set basis, then it should not be 
like this.

Finally: Is my approach reasonable at all or is that a bad idea? Does someone 
else use this method? I have not seen it published but then again I am new to 
this and many publications are a bit reserved on the topic of fitting strategy.

Thanks a lot and sorry for the long read,

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