  Inspired by Wannier's threshold law, we recognize that collision complex
decay meets the requirements of quantum-classical correspondence in
sufficiently exothermic ultracold reactions. We make use of this correspondence
to elucidate the classical foundations of ultracold reactions and to help bring
calculations currently beyond the capabilities of quantum mechanics within
reach. A classical method with a simplified model of many-body interactions is
provided for determination of the collision complex lifetime and demonstrated
for a reduced-dimensional system, as preliminary to the calculation of
collision complex lifetimes in the full-dimensional system.
