On 31/07/2013 03:07, Leonid Solovyov wrote:
... There is a
widely scattered belief that restraints always improve not only the
restrained values, but also the “unrestrained” part of structure such as
intermolecular distances, orientations, planarity etc.
Apart from your word "always", I certainly believe restraints can and
should improve the unrestrained output from Rietveld refinements. A
couple of points:
1) You should be checking whether the restraint target value is
consistent with the experimental data by looking at whether it comes
back at the end of the refinement. For example:
input restraint: C(sp3) to C(sp3) distance ~ 1.54
output distance: 1.54 -> OK
output distance: 1.4x -> Could it be sp2-sp2 or a wrong atom assignment?
An outlier in the fit to your restraints is just as much as a problem as
an outlier in the fit to your data. You should be looking at the
difference/esd, where the esd was also input.
2) If you are terribly worried what restraints are doing then you might
like to look into "Rfree" statistics. It would be cool for them to make
it into some mainstream Rietveld software
(doi:10.1524/zkri.219.12.791.55857, or doi:10.1038/nature12070).
So, assuming your restraints are correct for your sample then you are
adding information into your refinement. This causes the rest of the
results to improve as you reduce the noise in the output.
... The general
practice, however, consists in applying restraints in the cases of
highly biased reflection intensities and peak shape misfits when the
residuals are strongly correlated. In such cases restraints compensate
the influence of misfits on the restrained values at the expense of
their increased influence on the “unrestrained” part.
The restraints should improve the model, if they don't, see above.
Getting one part of a model right makes it easier to find out what is
missing to get the whole thing (about) right. If there are serious
misfits this means there is some problem to be resolved. Likelihood
methods can be useful at that point: again, it would be cool for those
to be more widely available.
Structure refinement is not likely to work out if you can't fit the peak
shape or have unidentified impurities or unfitted texture texture etc.
The Rietveld video game is about solving those problems to get to learn
something you didn't already know. It does not make sense to have the
atomic positions soaking up systematic problems.
The problem of having jammed the wrong molecule into a unit cell can be
identified by restraints which are violated. With rigid bodies it is
more difficult to find out what the data are trying to tell you. Is this
wrong molecule issue the problem you fear?
TL;DR: Garbage in garbage out. OK. What has that got to do with restraints?
Jon
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