[ccp4bb]: strange NCS/refinement problem (twinning?)

[Dima Klenchin <klenchin@facstaff.wisc.edu>]

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Hello all, 

I am facing a problem I am not sure how to address. 
how to approach (short of looking for a different xtal form). 

Good data set to 2A, indexes very well by P2 (67.947 76.813 98.441, 
beta=101.21) or P1 (alpha and gamma within 0.5 close to 90). 
Two molecules per cell. Scaled as P2, systematic absences indicate
P21 very clearly. 

Molrep finds what appears to be a good solution readily. Problems
start after that: When I restrain or constrain NCS during refinement, 
R free goes way up (R~30, Rfree > 40%). If I refine without NCS, 
R factors slip right away to 27/29 but this strange thing 
happens: one copy of the protein refines very well - low B factors,
very good looking map, two ligands totalling >100 non-H atoms
show up perfectly well on 1fofc map. A completely different 
story with another copy of the molecule: B factors are sky 
high and the map looks crappy with most of it being probably
model bias. Matthews coefficient is 6.1 with just one molecule...

At this point I have tried a lot of things hoping to find 
an error in previous steps and nothing shows up. Can this be
twinned crystal? Yates server does not appear to think so. 
Short of trying to find a new well-diffracting crystal form,
is there a reasonale solution? 

Thanks so much for any help, 

Dima