 the
  group in the direction (r-r0) where r0 = (x,y,z) - do not apply the force if
  the atom is within a distance delta of r0
for dragforce style, a xyz position of NULL means do not include that
  dimension in the distance or force computation
for shake style, certain bonds in the system are constrained every timestep
  to be at their equilibrium length, this is done by applying a SHAKE-like
  constraint to the forces on the atoms so that their position at the next
  timestep will preserve the atom separ