COMBINE <Parameter code> N(<Atom code>)
This statement creates a subset of the structure that will be treated as a rigid body. The positional shifts of each atom in this class are combined so that the relative positions of the atoms within the group are unaffected by the shifts to be applied. Several ridge bodies can be refined simultaneously by entering several COMBINE statements. Each COMBINE statement generates a single rigid body.
If non-positional parameters are combined, the shift applied to every atom is the average of the shift that would have been applied to each atom. Therefore if all of the atoms in the subset began with the same value then they would maintain values equal to each other, even though the particular value would change.
When positional parameters are combined the group is allowed to rotate and translate as a unit.
The details of this statement are described in the ``TNT Control File" chapter of the ``TNT Users' Guide".
CONSTANT <Parameter code> N(<Atom code>)
This statement sets the shifts for a specified subset of the structure to zero causing the specified parameters to remain constant.
The details of this statement are described in the ``TNT Control File" chapter of the ``TNT Users' Guide".