Dear All,
It has recently been put to me that 5-10% of reflections should always
be set aside for calculation of Rfree. However, when one has, say, 200
000 reflections (high resolution and/or large asymmetric unit), that
seems to me to be a waste, because it would mean removing 10 000 to 20
000 reflections from the refinement target.
My opinion is based on discussions with a particular knowledgeable
crystallographer and a meeting report ( http://journals.iucr.org/d/issues/1996/01/00/li0216/li0216.pdf
), in which 1000 reflections is said to be enough (i.e.
statistically valid). As a result, I always take 1000-1500
reflections, independently of the number of total reflections,
adjusting the percentage or the width of the thin shells (in case of
NCS) accordingly.
What is the opinion of the community? And, if you agree with me, how
should we try to get this opinion (more) generally accepted?
On a related note, how to refine a structure with only 5000
reflections, which could happen when you have a small a.u. and modest
resolution? Could, exceptionally, a lower absolute amount of
reflections be used for Rfree, say 500?
Greetings,
Mark
Mark J. van Raaij
Dpto de Bioquímica, Facultad de Farmacia
Universidad de Santiago
15782 Santiago de Compostela
Spain
http://web.usc.es/~vanraaij/
