A 3-phase equilibrium in argon is obtained by the thermodynamics of the perfect solid and liquid. The equation of state (EOS) for a perfect solid is obtained for a pure substance of spherical molecules that undergo molecular interaction of the Lennard-Jones form. The primitive internal energy EOS for a perfect solid (referred to as v0 EOS) is the sum of the thermally averaged kinetic energy and the potential energy of the nearest neighbors in a face-centered cubic (FCC) solid at 0 K. The extended internal energy EOS for a perfect solid (v1) includes a long-range effect in the low density region as the internal energy in the van der Waals EOS. The pressure EOS is written as the volume derivative of the potential energy at 0 K to satisfy the EOS with respect to thermodynamics. The temperature effect in the virial term is included in the extended pressure EOS. The EOS for a perfect liquid is the van der Waals EOS with empirical coefficients to explain the 3-phase equilibrium. The change in entropy for a reversible process is calculated by the standard method. The thermodynamic quantities of each phase are written as functions of volume and temperature. In this way, the Gibbs energy per molecule is plotted as a function of pressure for both solid and liquid phases, and the crossing point in the plot is the phase transition point. The p-V-T relations on the equilibrium lines are comparable with the experimental and molecular simulation results. The calculated average potential energy and entropy on the phase boundaries are consistent with the simulations. The thermodynamic quantities under a low pressure are compared with the molecular dynamic simulations. The quantities examined are volume, internal energy, enthalpy, entropy, Helmholtz energy, Gibbs energy, expansion coefficient, isothermal compressibility and heat capacity under a constant pressure.固体についてレナードージョーンズ相互作用と面心立方格子を仮定して、簡単化した状態方程式を示した。これを完全固体の状態方程式と呼ぶ。液体と気体については実験値の解析から得られたファンデルワールス状態方程式を仮定した。これを完全液体の状態方程式と呼ぶ。これらを用いて球形分子系の３相平衡をギブズエネルギーの計算から導いた。その結果をアルゴン系についての実験結果および、モンテカルロ法・分子動力学法シミュレーションと比較して、全体的に良い対応関係を得た
