| Résumé | The melting point and some liquid properties of ammonium dinitramide (ADN), NH4N(NO2)2, have been calculated using molecular dynamics calculations at various temperatures and pressures. The intramolecular potential for ADN was obtained from the AMBER 7 program and the intermolecular potential from Sorescu and Thompson [J. Phys. Chem. B 103, 6714 (1999)]. The simulations were performed for 3�2�4, 5�4�6, and 6�4�8 supercells of ADN. The 3�2�4 supercell was found to be adequate for predicting the melting point; however, the larger simulation cells were required to obtain converged results for the liquid properties. This model accurately predicts the temperature of the solid-to-liquid transition in ADN. The melting point of crystalline ADN has been determined by calculating the temperature dependence of the density, enthalpy, and radial distribution functions. The temperature dependence of the diffusion coefficient, calculated using equilibrium time-correlation functions, shows a discontinuity at the melting temperature and can also be used to determine the melting point. The value of the normal melting temperature of the perfect crystal calculated from the change in density is in the range 474�476 K, compared to the experimentally determined range 365�368 K. The difference is attributed to superheating of the perfect crystal. The superheating effect is eliminated by introducing voids in the crystal structure. Calculations of the temperature dependence of the density of a supercell with eight or more voids predict a melting temperature in the range 366�368 K, which is in excellent agreement with the experimental value. Melting temperatures have been calculated for pressures up to 0.8 GPa, which is the highest experimental pressure for ADN reported by Russell et al. [J. Phys. Chem. B 100, 3248 (1996)]. The computed dependence of the melting temperature on pressure is in excellent agreement with experiment. The temperature dependence of the diffusion and viscosity coefficient in the liquid temperature range were also calculated. |
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