  We study the threshold for chaos and its relation to thermalization in the 1D
mean-field Bose-Hubbard model, which in particular describes atoms in optical
lattices. We identify the threshold for chaos, which is finite in the
thermodynamic limit, and show that it is indeed a precursor of thermalization.
Far above the threshold, the state of the system after relaxation is governed
by the usual laws of statistical mechanics.
