Effect of monovacancies on the relative stability of fcc and hcp hard-sphere crystals

The effect of monovacancies on the free energy difference of close-packed hard-sphere crystals was examined. The monovacancy free energy and concentration were obtained via Monte Carlo simulations with a biased insertion method interpreted in a grand-canonical formalism. The difference in the effect of the vacancies to the difference between the free energies of the defect-free crystalline phases was compared. It was observed that the relative stability of stress-free fcc and hcp crystals is not affected by the presence of monovacancies, and fcc remaines the more stable phase over all solid-phase densities.open5

Optimized intermolecular potential for nitriles based on Anisotropic United Atoms model

An extension of the Anisotropic United Atoms intermolecular potential model is proposed for nitriles. The electrostatic part of the intermolecular potential is calculated using atomic charges obtained by a simple Mulliken population analysis. The repulsion-dispersion interaction parameters for methyl and methylene groups are taken from transferable AUA4 literature parameters [Ungerer et al., J. Chem. Phys., 2000, 112, 5499]. Non-bonding Lennard-Jones intermolecular potential parameters are regressed for the carbon and nitrogen atoms of the nitrile group (–C≡N) from experimental vapor-liquid equilibrium data of acetonitrile. Gibbs Ensemble Monte Carlo simulations and experimental data agreement is very good for acetonitrile, and better than previous molecular potential proposed by Hloucha et al. [J. Chem. Phys., 2000, 113, 5401]. The transferability of the resulting potential is then successfully tested, without any further readjustment, to predict vapor-liquid phase equilibrium of propionitrile and n-butyronitrile