Bart Hazes wrote:
Dale Tronrud wrote:
[EMAIL PROTECTED] wrote:
> Rotational near-crystallographic ncs is easy to handle this way, but
> what about translational pseudo-symmetry (or should that be
> pseudo-translational symmetry)? In such cases one whole set of
spots is
> systematically weaker than the other set. Then what is the
> "theoretically correct" way to calculate Rfree? Write one's own
code to
> sort the spots into two piles?
> Phoebe
>
Dear Phoebe,
I've always been a fan of splitting the test set in these situations.
The weak set of reflections provide information about the differences
between the ncs mates (and the deviation of the ncs operator from a
true crystallography operator) while the strong reflections provide
information about the average of the ncs mates. If you mix the two
sets in your Rfree calculation the strong set will tend to dominate
and will obscure the consequences of allowing you ncs mates too much
freedom to differ.
I haven't had to deal with this situation but my first impression is to
use the strong reflections for Rfree. For the strong reflections, and
any normal data, Rwork & Rfree are dominated by model errors and not
measurement errors. For the weak reflections measurement errors become
more significant if not dominant. In that case Rwork & Rfree will not be
a sensitive measure to judge model improvement and refinement strategy.
A second and possibly more important issue arises with determination of
Sigmaa values for maximum likelihood refinement. Sigmaa values are
related to the correlation between Fc and Fo amplitudes. When half of
your observed data is systematically weakened then this correlation is
going to be very high, even if the model is poor or completely wrong, as
long as it obeys the same pseudo-translation. If you only use the strong
reflections for Rfree I expect that should get around some of the issue.
Of course it can be valuable to also monitor the weak reflections to
optimize NCS restraints but probably not to drive maximum likelihood
refinement or to make general refinement strategy choices.
Bart
Dear Bart,
I agree that the way one uses the test set depends critically on the
question you are asking. In my letter I was focusing on that aspect
of the pseudo centered crystal problem where the strong/weak divide can
be used to particular advantage.
I have not thought as much about the matter of using the test set
to estimate the level of uncertainty in the parameters of a given model.
My gut response is that the strong/weak distinction is still significant.
Since the weak reflections contain information about the differences
between the two, ncs related, copies I suspect that a great many systematic
"errors" are subtracted out.
For example, if your model contains isotropic B's when, of course,
the atoms move anisotropically, your maps will contain difference features
due to these unmodeled motions. Since the anisotropic motions are
probably common to the two molecules, these features will be present in
the average structure described by the strong reflections but will be
subtracted out in the "difference" structure described by the weak
reflections. This argument implies to me that the strong reflections
need to be judged by the Sigma A derived from the strong test set and
the weak reflections judged by the weak test set.
Dale Tronrud
