Molecular dynamics simulations and self-consistent mean field (SCMF) calculations have been carried out for a relatively simple model for liquid ammonia. The model consists of Lennard-Jones spheres embedded with the dipole moment, the quadrupole moment and the point polarizability of the ammonia molecule. It is shown that the energy, pressure, dielectric constant and radial distribution function given by this model are in good agreement with experimental results for liquid ammonia. The SCMF theory is found to be much less accurate for this system than for the fluids of water-like particles studied previously. We conclude that many-body interactions which are not easily included in an effective pair potential are important for this model and perhaps for liquid ammonia.