  We analyze the coherent multi-mode dynamics of a system of coupled atomic and
molecular Bose gases. Starting from an atomic Bose-Einstein condensate with a
small thermal component, we observe a complete depletion of the atomic and
molecular condensate modes on a short time scale due to significant population
of excited states. Giant coherent oscillations between the two condensates for
typical parameters are almost completely suppressed. Our results cast serious
doubts on the common use of the 2-mode model for description of coupled
ultracold atomic-molecular systems and should be considered when planning
future experiments with ultracold molecules.
