topology {<dynamics-torsion-topology-statement>} end

<dynamics-torsion-topology-statement> :==

  reset 
  --{Resets the torsion topology. Should only
     be used stand-alone, i.e., TOPOlogy RESEt END.}

  fix torsion <atom-selection> <atom-selection> 
              <atom-selection> <atom-selection>
  --{Constrain the selected torsion angle.}
  fix group <atom-selection>
  --{Constrain all torsion angles within the selected group.}
  free torsion <atom-selection> <atom-selection>
               <atom-selection> <atom-selection>
  --{Un-constrain selected torsion angle.}
  free bond <atom-selection> <atom-selection> 
  --{Ignore the selected bond for torsion topology generation.
     The bond will still be restrained through the energy function, 
     but it is not constrained to a fixed value. All bonds that 
     match the double selection will be "free-ed". This statement
     is useful for breaking closed loops that are not part of
     rigid groups.}
  kdihmax=<real>
  --{Threshold for selecting torsions as free to rotate. 
     Rotation will be permitted around torsion angles with force constants
     less than this cutoff value. Torsion angles with force constants above
     this value will be constrained during dynamics.}

Notes:

The topology statements (except RESEt) are only parsed if no torsion topology
is already present.