Free-energy calculations of two crystalline phases of the molecular compound S-8 were performed via molecular dynamics (MD) simulations of these crystals. The elemental sulphur S-8 molecule model used in our MD calculations consists of a semiflexible closed chain, with fixed bond lengths and intramolecular 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 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(')-S-8 crystals are discussed. (C) 2003 American Institute of Physics.