We analyze two anisotropic atom-atom models used to describe the crystalline alpha, beta, and gamma phases of S-8 crystals, the most stable compound of elemental sulfur in solid phases, at ambient pressure and T less than or similar to 400 K. The calculations are performed via a series of classical molecular dynamics (MD) simulations, with flexible molecular models and using a constant pressure-constant temperature algorithm for the numerical simulations. All intramolecular modes that mix with lattice modes, and are therefore relevant to the onset of structural phase transitions, are taken into account. Comparisons with experimental data and previous results obtained with an isotropic atom-atom molecular model are also performed.