  In 2D bosonic systems ultra-soft interactions develop an interesting
phenomenology that ultimately leads to the appearance of supersolid phases in
free space conditions. While suggested in early theoretical works and despite
many further analytical efforts, the appearance of these exotic phases as well
as the detailed shape of the ground-state phase diagrams have not been
established yet. Here we develop a variational mean-field calculation for a
generic quantum system with cluster-forming interactions. We show that by
including the restriction of a fixed integer number of particles per cluster
the ground-state phase diagram can be obtained in great detail. The latter
includes the determination of coexistence regimes of crystals of different
occupancy as well as crystals with super-fluid phases. To illustrate the
application of the method we consider an ultrasoft potential for which the
computational phase diagram is known. Our results show very good quantitative
agreement with the simulations and suggest that the solid-superfluid
coexistence could be a reliable marker to locate supersolidity.
