The xrefin optimize group statement optimizes
grouped B-factors and occupancies, e.g.,
two for each residue. The
procedure minimizes the function 
. The Powell
conjugate gradient minimization method is used. B-factors
and occupancies can be refined simultaneously by specifying
both the B and the Q options.
Uniform shifts are applied
to the B-factors and occupancies for each of the selected
groups. The optimization is started using the current
values of the B-factors and occupancies, regardless of whether
these values are identical within a particular group.