Molecular dynamics studies of melting and liquid properties of ammonium dinitramide

摘要

The melting point and some liquid properties of ammonium dinitramide (ADN),NH_4N(NO_2)_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 3x2x4,5x4x6,and 6x4x8 supercells of ADN,The 3x2x4 supercell was found to be adequate for predicting the melting point;however,the larger simulation celsl were required to obtain converged results for the liqudi properties.This model accurately predicts teh temperature fo the solid-to-liquid transition in ADN,The melting point of crystaline AND has been determined by calculating the temperature ependence of the density,enthalpy,and radial distributin functions.The tempersture dependence of hte diffusion coefficient,calculated using equilibrium time-coreelation functions,shows a discontitinuity at the melting temperature and can also be usee 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 experimetnally eetermined 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 ependence fo fthe density fo a supercell with eight or more voids peridic a melting temperture in the range 366-368 K,which is in excellent agreement with the experimetnal value.melting temperature 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 temperature dependence of the melting temperature on pressure is in excellent agreement with experiment.The temperature dependence of the diffusion and viscosity coefficient in teh liquid temperature range were also calculated.