Randy Read wrote:
But there's another important issue: how many reflections do you need
to get a good estimate of the sigmaA values (as a function of
resolution) needed to calibrate the likelihood target? I know I've
discussed this in talks, but it doesn't look like I ever published
anything about it. Perhaps someone else has.
I did a bit on it in this paper...
K.Cowtan (2005) J. Appl. Cryst. 38, 193-198. Likelihood weighting of
partial structure factors using spline coefficients
http://journals.iucr.org/j/issues/2005/01/00/zm5022/zm5022.pdf
The number of reflections required varies with the level of error in the
model (i.e. with sigmaa). For refinement close to convergence, I could
use about 250 free reflections per sigmaA parameter (so 1500 would
probably do).
However when dealing with a very poor initial model, or, for example,
when using sigmaA in a density modification calculation, then it may be
necessary to use all the reflections.
