Free-energy calculations of elemental sulphur crystals via molecular dynamics simulations

Abstract

Free-energy calculations of two crystalline phases of the molecular compound S8 were performed via molecular dynamics (MD) simulations of these crystals. The elemental sulphur S8 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 α– and α′–S8 crystals are discussed.