[RMCProfile-users] Weighting of data sets

[Chris Kerr]

If the Bragg signal is very weak I would recommend not using it at all; or using a low 
QMAX / high DMIN that only includes the largest peaks; this will drastically reduce the 
amount of CPU time used.

In version 6.x the WEIGHT keyword, as you have noticed, counterintuitively gives a 
stronger weighting when the value of the keyword is smaller.

The weight keyword should be set so that, once you have finished the initial 
convergence stage (eliminating e.g. the sharp peaks in the G(r) of an initial configuration 
based on the long-range average structure), a typical move gives a change on the order 
of +/- 1 in the overall chi^2.

On Tuesday 03 Jun 2014 20:24:03 Henrik Mauroy wrote:
> Hi everyone.
> This might be a dumb question but I wonder how the weighting of the data
> sets are done. So far I've used equal weighting for G(r), F(Q) and Bragg
> and haven't been troubled by this yet.
> I have some data collected at GEM where the Bragg data is completely
> impossible to fit due to a unusually weak Bragg signal (due to
> cancelling negative scattering lengths), and need to turn the weighting
> of the Bragg data to such a low number that it is not being part of the
> fitting.
> 
> 1) What effect does a higher number for the weight do?
> 2) In what range should the weight-number be? In most examples it is
> smaller than 0.1, but can you use bigger or smaller numbers?
> 3) I see from experimenting with high and low numbers that the chi^2
> value calculated for the data set in question becomes larger for a lower
> weighting. Is this the number RMCProfile is trying to reduce?
> 
> Thanks for any answer!
> 
> Cheers,
> Henrik
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> rmcprofile-users at rmcprofile.org
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