And another (not so) minor point: Defferences between R/Rfree with
twin on and off suggest that there is strong correlation between twin
and NCS. For this cases better dealt with if you use twin refinement
with sufficiently strong NCS restraints. In new refmac (www.ysbl.york.ac.uk/refmac/
site: look for the experimental version) and in phenix (as far as I
know) you can do NCS automatically.
Even after twin refinement with ncs I would check if higher space
group is possible. You can try to do it using for example zanuda (from
the webserver www.ysbl.york.ac.uk/YSBLPrograms/index.jsp). I think
phenix.triage may be able to do similar things.
regards
Garib
On 23 Apr 2010, at 22:28, hari jayaram wrote:
I am refining a twinned dataset in possible spacegroup P212121 .
Pointless thinks it is P43212 , but based on reading this posting (http://www.phenix-online.org/pipermail/phenixbb/2007-September/000501.html
) I think it is P212121.
The starting R/Rfree after molecular replacement ( single site
mutant) was 34/38 to 2.2 A
After an initial round of restrained refinement ( without twin
refinement) and minimal rebuilding I got the R/Rfree to 30/34
Then I did an amplitude based twin refinement - The twin fraction
was 0.48 k,h,-l and 0.52 h,k,l and The r/rfree became 24/29
After a little more rebuilding ( a few residues out of 800 residues
in ASU) and another twin refinement I got an r/rfree of 22/27 . Now
the twin fraction was 0.87 (h,k,l) and 0.13 (k,h,-1)
The maps looked a little better allowing me to fix a few more residues
Finally the same twin refinement gives me no twin operators and the
R/Rfree is 22/26
All the twinning tests indicate a serious twinning in my crystal.
Any ideas why I am seeing this
Hari
