very low B values

[Zbyszek Otwinewski]

Very low B values may be artefact of data reduction programs.
Some people/some programs delete weak reflection during data reduction/scaling.
Some programs (e.g. XDS) do not even give a choice.
Deleting weak obseravtions (for example negative) will make high res.
data stronger on average than their real values.
Another factor that can make high res. data artificially strong is
use of wrong data as a reference in scaling.
Deleting weak data will not make all B factors negative, but it will reduce
apparent B-factors. If there is natural large spread of B-factors in the
crystal structure, well ordered atoms may refine to negative B-factors.

Zbyszek Otwinowski