one more change

.. note::

   The cost of each energy evaluation is essentially the cost of an MD

   timestep.  Thus invoking this fix for a 3d system once has a cost

   timestep.  Thus invoking this fix once for a 3d system has a cost

   of 6N timesteps, where N is the total number of atoms in the system

   (assuming all atoms are included in the group).  So this fix can be

   very expensive to use for large systems.