clarify docs on what energy is monitored

zero or more keyword/value pairs may be appended

keyword = {emax} or {units} :ul

  {emax} value = Emax

    Emax = maximum energy change for an atom in one timestep (energy units)

    Emax = maximum kinetic energy change for an atom in one timestep (energy units)

  {units} value = {lattice} or {box}

    lattice = Xmax is defined in lattice units

    box = Xmax is defined in simulation box units :pre

Reset the timestep size every N steps during a run, so that no atom

moves further than Xmax, based on current atom velocities and forces,

and (optionally) no atom's energy changes more than Emax.

and (optionally) no atom's kinetic energy changes by more than Emax.

This can be useful when starting from a configuration with overlapping

atoms, where forces will be large.  Or it can be useful when running

an impact simulation where one or more high-energy atoms collide with

is generated.  The estimate is conservative in that the atom's

displacement is guaranteed not to exceed {Xmax}, though it may be

smaller. Also, if the {Emax} value is given, for each atom, the

timestep is limited to a value that allows the atom's energy to change

by at most {Emax}.

timestep is limited to a value that allows the atom's kinetic energy

to change by at most {Emax}.

Given this putative timestep for each atom, the minimum timestep value

across all atoms is computed.  Then the {Tmin} and {Tmax} bounds are

[Default:]

The option defaults is units = lattice, and no energy change limit.

The option defaults is units = lattice, and no kinetic energy change limit.