It is often desirable to have the option of holding portions of the structure fixed, or constraining a group to move as a rigid body (e.g. Sussman, Holbrook, Church & Kim, 1977). Because these options do not depend on the nature of the function being minimized they have been implemented in the control program.
The control program also allows one to limit the range of values that the temperature factors and occupancies can assume and, in addition, to constrain a group of atoms to have the same temperature factor or occupancy (e.g. to allow for partial occupancy by an inhibitor).