The LINK statement was created to define these types of links. The form of the statement is quite simple:
LINK <Residue name> <Target name> <Link type>
The statement merely contains the name of the two residues and the type of link between them. You do not have to specify the symmetry operator, TNT will determine that by itself.
What would the LINK statement look like if we had a protein where residue
66 was linked by a disulfide bridge to its image across a two-fold? Simply
``LINK 66 66 SS''.