  The ground-state phase diagram of a two-dimensional Bose system with
dipole-dipole interactions is studied by means of quantum Monte Carlo
technique. Our calculation predicts a quantum phase transition from gas to
solid phase when the density increases. In the gas phase the condensate
fraction is calculated as a function of the density. Using Feynman
approximation, the collective excitation branch is studied and appearance of a
roton minimum is observed. Results of the static structure factor at both sides
of the gas-solid phase are also presented. The Lindeman ratio at the transition
point comes to be $\gamma = 0.230(6)$. The condensate fraction in the gas phase
is estimated as a function of the density.
