(I'm probably wrong, but I want someone to show me,and not with
hand-waving
arguments or invocation of crystallographic intuition or such)
To convince me, someone needs to show that the expected value of the
change
in |Fo-Fc| at a test reflection upon a change in the model (a step of
refinement)
is negative, even in the absence of any real improvement in the model,
simply because the change reduces |Fo-Fc| at a sym-related working
reflection.
The problem is that a) the statistical drift will be very small, and b)
that this will be for *almost every* reflection in the test set. If it
were just a few, you'd be right, but not when it's all of them: then
your Rfree will not be informative.
That's in the absence of NCS restraints. In their presence, it's bad
anyway, because you're forcing Fc to be (almost) equal for both reflections.
phx.
