I agree with Gerard. Example: it's unlikely to achieve a result of
rigid-body refinement (when you refine six rotation/translation
parameters) by replacing it with refining individual coordinates using
infinitely large weights for restraints.
Pavel.
On 9/22/10 1:46 PM, Gerard DVD Kleywegt wrote:
Hi Ian,
First, constraints are just a special case of restraints in the limit
of infinite weights, in fact one way of getting constraints is simply
to use restraints with very large weights (though not too large that
you get rounding problems). These 'pseudo-constraints' will be
indistinguishable in effect from the 'real thing'. So why treat
restraints and constraints differently as far as the statistics are
concerned: the difference is purely one of implementation.
In practice this is not true, of course. If you impose "infinitely
strong" NCS restraints, any change to a thusly restrained parameter by
the refinement program will make the target function infinite, so
effectively your model will never change. This is very different from
the behaviour under NCS constraints and the resulting models in these
two cases will in fact be very easily distinguishable.
--Gerard
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Gerard J. Kleywegt
Dept. of Cell & Molecular Biology University of Uppsala
Biomedical Centre Box 596
SE-751 24 Uppsala SWEDEN
http://xray.bmc.uu.se/gerard/ mailto:ger...@xray.bmc.uu.se
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