[ccp4bb]: refinement weights

[Bernard Santarsiero <bds@uic.edu>]

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While we're on the subject of refinement, with REFMAC, CNS, TNT, and
SHELX, what do people do to adjust the relative weights.  

In CNS I typically refine with "wa=-1," which sets up a reasonable
relative weight between the geometry terms and the xray terms for an
incomplete model.  Towards the end of refinement, I double or triple
the value found with "wa=-1." I've found that with "wa=-1" various
programs like whatcheck and procheck find that the restraints are too
tight.  When I increase wa, then both the R(cryst) and R(free) decrease a
bit; at some point R(free) doesn't drop anymore, even if R(cryst) does, so
that's a reasonable place to stop.  I also look at the rmsd on bond
lengths (0.01 or lower as a target), and on bond angles (1.5 or lower as a
target).  

In SHELX, I leave the weight as 0.2, and never mess with it, even with
small molecule structures.  The sigma's of the intensities seem to be
correct from various packages (denzo, bioteX, mosflm), since the small
molecule structures gives GOF's close to 1.0 without the additional fudge
(weight adjustment) in SHELX.  Since I don't know what the systematic
errors are in the model or the data, I don't believe that the GOF's should
be close to 1.0, unless you know you have excellent data and an excellent
model.  

I haven't used REFMAC or TNT in awhile, so I'm not sure how the weighting
schemes are adjusted.

Bernie Santarsiero