We present Quantum Monte Carlo simulations of the soft-core bosonic Hubbard
model with a ring exchange term K. For values of K which exceed roughly half
the on-site repulsion U, the density is a non-monotonic function of the
chemical potential, indicating that the system has a tendency to phase
separate. This behavior is confirmed by an examination of the density-density
structure factor and real space images of the boson configurations. Adding a
near-neighbor repulsion can compete with phase separation, but still does not
give rise to a stable normal Bose liquid.Comment: 12 pages, 23 figure