This statement optimizes individual (restrained) isotropic B-factors. Distinctions between backbone and side-chain B-factor restraints can be made by appropriate selections. B-factors of atoms are both selected in xrefin and free to move. The procedure minimizes the target function
where the restraining term 
is given by
The summations are carried out over all bond and angle
terms present in the molecular structure that involve
selected and free atoms. The last sum
is used only if non-crystallographic symmetry restraints
are present (see Chapter 16). The Powell
conjugate gradient minimization method is used.