  A very simple explicit integrator for the rotational motion of rigid linear
molecules is presented which can preserve the rigidity of the molecules without
requiring any constraint force. The integrator is time-reversible and
symplectic, thus preserving volume in phase space. It also conserves angular
momentum. As expected, having all these virtues, it remains stable for large
time-steps. Both the leap-frog and velocity-Verlet versions of the integrator
are described. Since it features angular velocities explicitly, the integrator
can be conveniently coupled to different thermostats. As a specific example,
the Nose-Hoover thermostatting is discussed in detail to aid ready
implementation. A simpler and faster adaptation of the main integrator,
appropriate for double precision computing, is also offered.
