  We study the Lyapunov instability of a two-dimensional fluid composed of
rigid diatomic molecules, with two interaction sites each, and interacting with
a WCA site-site potential. We compute full spectra of Lyapunov exponents for
such a molecular system. These exponents characterize the rate at which
neighboring trajectories diverge or converge exponentially in phase space.
Quam. These exponents characterize the rate at which neighboring trajectories
diverge or converge exponentially in phase space. Qualitative different degrees
of freedom -- such as rotation and translation -- affect the Lyapunov spectrum
differently. We study this phenomenon by systematically varying the molecular
shape and the density. We define and evaluate ``rotation numbers'' measuring
the time averaged modulus of the angular velocities for vectors connecting
perturbed satellite trajectories with an unperturbed reference trajectory in
phase space. For reasons of comparison, various time correlation functions for
translation and rotation are computed. The relative dynamics of perturbed
trajectories is also studied in certain subspaces of the phase space associated
with center-of-mass and orientational molecular motion.
