Thermodynamics and Molecular Dynamic Simulations of Three-phase Equilibrium in Argon (v5)

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Equations of state (EOS) are proposed for a system involving argon and consisting of a perfect solid and a perfect liquid composed of single spherical molecules in which Lennard–Jones interactions are assumed. Molecular dynamics simulations of this system were performed to determine the temperature and density dependencies of the internal energy and pressure and the supercooled liquid state was also examined. The sum of the average kinetic and potential energies at 0 K and the temperature-dependent potential energy was applied as the internal energy term in the EOS, while the temperature-dependent term of the average potential energy was assumed to be a linear function of the temperature and its coefficient was expressed as a polynomial function of the number density. The pressure was expressed in a similar manner, such that it satisfied the thermodynamic EOS. Using this approach, the equilibrium condition was solved numerically for the phase equilibrium of argon. The Gibbs energy thus calculated gives a reasonable transition pressure for argon's three-phase equilibrium state. The thermodynamic properties at low pressures were found to exhibit significant temperature dependencies.