  We analize two anisotropic atom-atom models used to describe the crystalline
alpha,beta and gamma phases of S8 crystals, the most stable compound of
elemental sulfur in solid phases, at ambient pressure and T<=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 on
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.
