  Estimates for the lifetime of collision complexes formed during ultracold
molecular collisions based on density-of-states arguments are shown to be
consistent with similar estimate based on classical trajectory calculations. In
the classical version, these collisions are shown to exhibit chaos, and their
fractal dimensions are calculated versus collision energy. From these results,
a picture emerges that ultracold collisions are classically ergodic, justifying
the density-of-states estimates for lifetimes. These results point the way
toward using the techniques of classical and quantum chaos to interpret
molecular collisions in the ultracold regime.
