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[ccp4bb]: TLS refinement: Fix B-factors?



Dear all,
 
I am refining a structure with two protomers in the asymmetric unit; resolution range 25-2.0 Angstroem; hexagonal space group P6122; total number of reflections: 22 000; total number of atoms to refine: ~2800.
 
I have been using TLS refinement in the last stage of refinement and got a nice drop in Rfree and Rcryst: before TLS: Rfree=19.3 Rcryst=17.3. after TLS refinement: Rfree=18.2 Rcryst=16.6. I used two TLS groups for the two protomers in the asymmetic unit respectively, 5 cycles of TLS refinement+9 cycles of normal Refmac refinement. However I did not turn the "Fix B-factors" field on, though I entered Wilson B in the respective field. When going back and runnning the same refinement procedure as above, this time fixing B-factors to Wilson B I get: Rfree=18.5 Rcryst= 17.3. i.e little worse than above.
 
Since I am not very experienced in using TLS refinement my question is whether it is necessary to fix the B-factors. It makes more sense to me, but I would like to have some competent advice on that. If fixing is advicable: to what should I fix it  (with this particular refinement problem).
 
 
Thanks very much,
 
Florian
 
-------------------------------------
Florian Schmitzberger
University of Cambridge
Department of Biochemistry
80 Tennis Court Road
CB2 1GA Cambridge
Tel: 0044-1223-766029