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When (trying) to refine a DNA-structure (resolution 2.5) using Refmac_5.5.0072 (CCP4 6.1.1), some of the H-bonds between Watson-Crick bases are becoming too large.

Reducing the Matrix weighting term to tighten the geometry, doesn't effect these H-bond distances much.

Reducing the "VDW SIGMA HBOND" also doesn't solve the problem.

Adding "external distance restraints" does the trick, but some B-factors (not the ones involved in H-bonding!) blow up completely.

Hence, what's actually the best way of tighten H-bond restraints in Refmac, or am I overlooking some other issues here..?

Thank you very much.

Regards

Kristof Van Hecke