 outside of which to apply force (in distance units)
         shake
                (1) max # of SHAKE iterations within each atom cluster
                (2) SHAKE tolerance (accuracy of one part in tolerance)
                (3) print bond statistics every this many steps (0 = never)

define a constraint
cannot skip a constraint number, all must be used before a run is performed
use appropriate number of coeffs for a particular style
which atoms or bonds the constraint will affect is set by the
  "assign 