  We investigate the crystal structure of classical systems of spherical
particles with an embedded point dipole at T=0. The ferroelectric ground state
energy is calculated using generalizations of the Ewald summation technique.
Due to the reduced symmetry compared to the nonpolar case the crystals are
never strictly cubic. For the Stockmayer (i.e., Lennard-Jones plus dipolar)
interaction three phases are found upon increasing the dipole moment:
hexagonal, body-centered orthorhombic, and body-centered tetragonal. An even
richer phase diagram arises for dipolar soft spheres with a purely repulsive
inverse power law potential $\sim r^{-n}$. A crossover between qualitatively
different sequences of phases occurs near the exponent $n=12$. The results are
applicable to electro- and magnetorheological fluids. In addition to the exact
ground state analysis we study freezing of the Stockmayer fluid by
density-functional theory.
