  Free energy calculations of two crystalline phases of the molecular compound
S8 were performed via molecular dynamics simulations of these crystals. The
elemental sulphur S8 molecule model used in our MD calculations consists of a
semi-flexible closed chain, with fixed bond lengths and intra-molecular
potentials for its bending and torsional angles. The intermolecular potential
is of the atom-atom Lennard-Jones type. Two free energy calculation methods
were implemented: the accurate thermodynamic integration method proposed by
Frenkel and Ladd and an estimation that takes into account the contribution of
the zero point energy and the entropy of the crystalline vibrational modes to
the free energy of the crystal. The last estimation has the enormous advantage
of being easily obtained from a single MD simulation. Here we compare both free
energy calculation methods and analyze the reliability of the fast estimation
via the vibrational density of states obtained from constrained MD simulations.
New results on alpha- and alpha'- S8 crystals are discussed
