THE STRUCTURE AND DYNAMICS OF SOLID BENZENE .2. MOLECULAR-DYNAMICS STUDIES

Extensive molecular dynamics (MD) simulations are reported for benzene-d6, based on the potential of Karlstrom et al. [J. Am. Chem. Soc. 105, 3777 (1983)], linearly rescaled. Three phenomena have been identified as a function of temperature, a solid state structural phase transition, a lattice instability, and melting. The simulations clarify that the first two phenomena are distinct from premelting; they rationalize some of our recent experimental data, and lead to a suggestion for further experiments to substantiate these phenomena. Rescaling the ab initio potential gave lattice parameters and an internal energy in very good accord with experiment across a wide temperature range. The MD lattice instability at 335 K was shown not to be true melting, the melting point being found at 272 K using a system with free surfaces. This result, the latent heat and volume discontinuity at melting, and the thermal motion amplitudes were all found to be in very good agreement with experiment. The first high-pressure phase of benzene was also successfully modeled by the potential, giving excellent values for the structural parameters. A MD result not found in experiment is a disordering transition in which the distinction between the orientations of pairs of molecules in the unit cell is lost. Experimental data appears to show that the same effect would be seen in real benzene were the solid not to melt first. It has also been shown that the application of moderate pressures may make the transition more apparent experimentally.