fix" command
all of the constraints (except for rescale) are applied every timestep
all specified temperatures are in temperature units
for style setforce, a coeff of NULL means do not alter that force component
for style aveforce, average force on the group of fixed atoms is computed,
  then new average force is added in and actual force on each atom is set
  to new total value -> has effect of applying same force to entire group
  of atoms
thermostatting constraints (rescale, hoover/drag, langevin) cannot