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

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

  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 topology generation.
     The bond will be restrained and not constrained during the dynamics.}
  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.}
  maxtree=<integer>
  --{Maximum number of trees allocated. This is the number of independent,
     ie. unconnected bodies in the system.}
  maxchain=<integer>
  --{Maximum number of chains per tree. A chain is a connected set of
     unbranched torsion angles.}
  maxlength=<integer>
  --{Maximum length of a chain in a tree. ie. the number of torsion angles
     in an unbranched chain.}
  maxbond=<integer>
  --{Maximum number of bonds per atom.}
  maxdihe=<integer>
  --{Maximum number of torsions per atom.}
  maxjoint=<integer>
  --{Maximum number of branch points per constrained group.}