Dear Theresa,
all types of twinning - merohedral, pseudo-merohedral and non-merohedral
ones - can be solved given somewhat favorable conditions. In small
molecule crystallography, it's quite common, especially for (pseudo)
merohedral twins, it becomes increasingly popular for macromolecules as
well. However, if your crystal is a split one with many domains, you
should possibly look for a better crystal.
You need to treat your data correctly:
- In cases of (pseudo) merohedral twinning: Integrate in the lower -
correct - space group. Not all methods of structure solution work well
with twins, especially S-SAD - which is very sensitive towards data
errors - and those methods that require several crystals. For
refinement, the free R set has to contain also reflections related by
twinning, and the random R value will go down, so don't be fooled.
- In case of a non-merohedral twin, particular attention should be paid
to overlapping reflections, they should be left out at first for
indexing and treated special in integration. Scaling is also more
difficult. While non-merohedral twins can be dealt with, it is not
(yet?) common practice for macromolecules.
Best wishes
Andrea.
Am 22.02.2012 19:56, schrieb Theresa H. Hsu:
Hi all.
I have a 'learning' question based on recent thread where crystal twinning is
mentioned. With the current computational methods, what types of twinnig can
and cannot be solved with computers?
Thank you.
Theresa
ath...@shelx.uni-ac.gwdg.de http://shelx.uni-ac.gwdg.de/~athorn
