Thermodynamics and Molecular Dynamic Simulations of Three-phase Equilibrium in Argon (v8)

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Equations of state (EOSs) are proposed for a system consisting of a perfect solid and a perfect liquid made up of single spherical molecules. The Lennard-Jones interaction is assumed for this system. Molecular dynamics simulations are performed in order to determine the temperature and density dependences of the internal energy and pressure. The supercooled liquid state is also examined. The internal energy term in the EOSs is the sum of the average kinetic and potential energies at 0 K and the temperature-dependent potential energy. The temperature-dependent term of the average potential energy is assumed to be a linear function of temperature, and its coefficient is expressed as a polynomial function of the number density. The pressure is expressed in a similar manner, where the pressure satisfies the thermodynamic EOS. The equilibrium condition is solved numerically for the phase equilibrium of argon. The Gibbs energy provides a reasonable transition pressure for three-phase equilibrium in argon. The thermodynamic properties at low pressures have significant temperature dependences. The linear character of the pressure and internal energy as functions of temperature in the condensed phases is discussed based on the short-term vibration motion.