  The high-barrier quantum tunneling regime of a Bose-Einstein condensate
confined in a ring-shaped optical lattice is investigated. By means of a change
of basis transformation, connecting the set of `vortex' Bloch states and a
Wannier-like set of localized wave functions, we derive a generalized
Bose-Hubbard Hamiltonian. In addition to the usual hopping rate terms, such a
Hamiltonian takes into account interaction-driven tunneling processes, which
are shown to play a principal role at high filling factors, when the standard
hopping rate parameter turns out to be negative. By calculating the energy and
atomic current of a Bloch state, we show that such a hopping rate must be
replaced by an effective hopping rate parameter containing the additional
contribution an interaction-driven hopping rate. Such a contribution turns out
to be crucial at high filling factors, since it preserves the positivity of the
effective hopping rate parameter. Level crossings between the energies per
particle of a Wannier-like state and the superfluid ground state are
interpreted as a signature of the transition to configurations with
macroscopically occupied states at each lattice site.
