This is a very common thing - twinning is a sort of accidental overlap
of crystal fragments and thus will be different for diferent
"crystals". ( Should a twinned object be called a crystal? Discuss)
More serious - it is probably different for different parts of the data
collection if your crystal is smaller than the beam - I dont know if
anyone has ever tried to analyse for this.
In our experience smaller is often better..
Eleanor
Mark Mayer wrote:
For cases where people have had merohedral twinning, did the twin fraction vary substantially
between individual crystals grown under indentical conditions? I have no prior experience with
merohedral twinning, and was surprised to see that the twin fraction varied substantially as detailed
below, and that by screening we were able to get untwinned xtals.
The project started with a weak home data set for which the twin fraction was 0.478, and which
scaled in both H3 and H32. We just came back from APS with data sets from another three crystals,
for which the ML twin fraction, estimated using phenix.xtriage with scalepack merged intensities as
input, varied from 0.335, 0.219 and 0.02. The latter is refining very nicely, in H3 and will not scale in
H32.
Thanks - Mark
