Yuan,
Did you permute your axis in orthorhombic so your screw axis is in
the right place? Did you decide on the screw axis according to
systematic absences? You could permute the axis and try to merge in
monoclinic, but I would not recommend it if the data actually has
pseudomerohedral twinning. The two crystals will likely have differing
twin fractions introducing more error. What you need is a new complete
dataset from a single crystal if the space group is monoclinic. The
large drop in R-factor is a strong indication that the data is twinned
as long as your model is pretty accurate. Your twin fraction is probably
going to be close to 0.5 if you could merge your data in orthorhombic so
"detwinning" would not be possible (and I wouldn't recommend doing it
anyway). The way Phenix.refine refines a model with a high twin fraction
is different (proportionality rules) which can introduce model bias, so
your model should be pretty close to the end product. I recommend
refining your structure as well as you can without inputting the twin
law until the very end.
Jon Schuermann
--
Jonathan P. Schuermann, Ph. D.
Beamline Scientist
NE-CAT, Building 436E
Advanced Photon Source (APS)
Argonne National Laboratory
9700 South Cass Avenue
Argonne, IL 60439
email: schue...@anl.gov
Tel: (630) 252-0682
Fax: (630) 252-0687
