Symmetric linkages are referred to covalent bonds, bond angles
and dihedral angles involving atoms in different but
symmetrically related subunits. The *symmetric_linkage*
statement is used to create a data base describing the symmetric
linkages between symmetric related subunits for potential energy
calculations. After the symmetric linkages are defined, the
non-bonded interactions between atoms linked by the symmetric
linkages become invalid. These non-bonded interactions must be
removed from the table of non-bonded intersubunit interactions.
Exclusion of these invalid non-bonded interactions should be
defined in the data base using either **AUTOexclude** or **EXCLude** statements.

The symmetric_linkage data base is not stored in the structure
file (PSF file). Therefore, the *symmetric_linkage-statements* have to be entered every time when
the information is required for the non-bonded energy
calculations.