Thermodynamics of Three-Phase Equilibrium in Argon Based on a Perfect Solid and a Perfect Liquid (v4)

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Equations of state (EOS) 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 to determine the temperature and density dependences of the internal energy and pressure. The internal energy term in the EOS 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 the temperature and its coefficient is expressed as a polynomial of the number density. The pressure is expressed in a similar way, where the pressure satisfies the thermodynamic EOS. The equilibrium condition is solved numerically for the phase equilibrium of argon. The Gibbs energy gives a reasonable transition pressure for three-phase equilibrium in argon. The thermodynamic properties at low pressures have significant temperature dependences.