SYMMetric_linkage { $<$symmetric_linkage-statement$>$ } END
This statement must be invoked from the main level of X-PLOR.
BOND $<$atom-spec$>$ $<$atom-spec$>$
defines a covalent bond between two atoms.
ANGLe $<$atom-spec$>$ $<$atom-spec$>$ $<$atom-spec$>$
defines a bond angle.
DIHEdral_angle $<$atom-spec$>$ $<$atom-spec$>$ $<$atom-spec$>$

defines a dihedral angle.
excludes (1-2, 1-3 and 1-4) invalid non-bonded interactions cause by the symmetric linkages.
EXCLude $<$atom-spec$>$ $<$atom-spec$>$ ... END
defines exclusion of non-bonded interactions between first atom and the following atoms in the statement.
NCS=$<$internal non-crystallographic symmetry operation ID$>$ ATOM=$<$atom selection$>$ (see X-PLOR Section 2.15)
THREshold $<$real$>$
defines a threshold value for monitoring the phi-psi conformation energy in energy minimization or molecular dynamics calculation (default 4.0 Kcal/mol).

Xplor-NIH 2023-11-10