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Libertador 8250, (1429) Buenos Aires, Argentina. e-mails: clopasto@cnea.gov.ar and gamba@cnea.gov.ar S 8 is the most stable compound of elemental sulfur in solid and liquid phases, at ambient pressure and below 400K. Three crystalline phases of S 8 have been clearly identified in this range of thermodynamic parameters, although no calculation of its phase diagram has been performed yet. α-and γ-S 8 are orientationally ordered crystals while β-S 8 is measured as orientationally disordered. In this paper we analyze the phase diagram of S 8 crystals, as given by a simple and flexible molecule model, via a series of molecular dynamics (MD) simulations. The calculations are performed in the constant pressure-constant temperature ensemble, using an algorithm that is able to reproduce structural phase transitions. Introduction:Elemental sulfur shows a rich variety of molecular forms and therefore of chemical and physical properties in the crystalline and liquid phases 1,2 . Recently, a high pressure superconductor transition has been found, with T c =17K (at 162GPa), the highest known for any elemental solid 3 . Natural sulfur is of relevance in geophysics, astrophysics, material sciences and also of massive industrial use 1 . Nevertheless, the complex phase diagram and properties of its condensed phases are far from well known 1,2 .The most stable sulfur allotrope at ambient temperature and pressure (STP) is S 8 , a crown-shaped cyclic S 8 molecule, stable in solid, liquid and gas phases. 1Orthorhombic α-S 8 is the crystalline form stable at STP. Figs 1a shows its "crankshaft" structure with four molecules in the primitive unit cell. The space group is D 24 2h (Fddd), with 16 molecules in this non-primitive orthorhombic cell. If α-S 8 is slowly heated, it shows a phase transition to monoclinic β-S 8 at 369K (Fig. 1b), which melts at 393K 1 . Nevertheless, most α-S 8 crystals do no easily convert to β-S 8 , they melt, instead, at 385.8K 1,2 . β-S 8 is a monoclinic crystal, with six molecules in the primitive unit cell, two of them orientationally disordered 4,5 . A third crystalline allotrope has been observed: γ-S 8 (Fig. 1c), that can be obtained from solutions of S 8 or from its melt 1,2 . The space group is C 4 2h (P2/c), with four molecules in the pseudo-hexagonal closed-packed unit cell 6 . The density of this allotrope, at STP, is 5.8 % higher than that of α-S 8 .In spite of its relevance, calculations on the crystalline phase diagram of elemental sulfur molecules are scarce. Previous molecular dynamics (MD) simulations of crystalline S 8 were limited to constant-volume simulations at normal pressure and temperatures of α-S 8 7 , with the only aim to study the intramolecular frequencies obtained by Raman and ir. measurements. The same scope was followed by numerous works that measured crystalline frequencies and performed calculations based on lattice dynamics sums in different aproximations 8 .No attempt to reproduce the phase diagram of this molecule, including determination of crystalline structures, lat...

e-mails: clopasto@cnea.gov.ar and gamba@cnea.gov.ar α-S 8 is the most stable crystalline form, at ambient pressure and temperature (STP), of elemental sulfur. In this paper we analyze the zero pressure low temperature part of the phase diagram of this crystal, in order to test a simple and flexible model molecule. The calculations consist in a series of molecular dynamics (MD) simulations, performed in the constant pressureconstant temperature ensemble. Our calculations show that this model, that gives good results for three crystalline phases at STP and T > ∼ 300K, fails at low temperatures, predicting a structural phase transition at 200K where there should be none.

An atom–atom Lennard-Jones (LJ) model for the intermolecular potential of carborane molecules (B10C2H12) is developed and used in a series of classical constant-pressure molecular dynamics simulations of p-, m- and o-carborane crystals, at several temperatures and zero pressure. The potential model reproduces many structural and dynamical properties of these crystals, in particular the high temperature orientationally disordered plastic phase common to the three compounds. This work stresses the need of further experimental data to improve the potential model.

The statical properties and lattice dynamics of the two known phases of crystalline acetylene have been studied, using a model which takes account of electrostatical intermolecular as well as atom–atom interactions. Good agreement with observed statical and dynamical properties of both phases has been achieved. The calculated phonon dispersion curves and group symmetry considerations led us to the identification of the phonons in both phases which are possibly associated with the structural phase transition.