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:[log in to unmask]
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