Molecular force field optimization and molecular simulation of vapor–liquid phase equilibrium for oxygen

Abstract

As one of the low-temperature fixed-points defined by the International Temperature Scale, the vapor–liquid phase equilibrium properties of oxygen is essential in the field of low temperature measurement. In order to better explore its microscopic vapor–liquid phase equilibrium mechanisms, a high-precision force field model is required. Therefore, this paper focuses on the development and optimization of a molecular force field for oxygen. Firstly, the all-atomic force field for oxygen was constructed based on first principles. On this basis, Gibbs ensemble Monte Carlo molecular simulations were conducted from the triple point to the critical point region. The simulation results of saturated liquid density, saturated vapor density, saturated vapor pressure, and evaporation enthalpy exhibited average absolute relative deviations of 0.56%, 3.48%, 4.97%, and 1.56% compared to the calculations by the REFPROP software, which are better than those obtained using other force fields previously reported. Secondly, a molecular force field coupling optimization method was proposed. By qualitatively analyzing the impact of each term in the different potential function of oxygen, the optimal terms were coupled to obtain the most accurate force field. The average absolute relative deviations of the corresponding simulation results were optimized to 0.52%, 2.92%, 3.87%, and 1.37%, which can serve as a reference for optimizing other force fields. Finally, vapor–liquid phase equilibrium simulations were conducted for the binary mixture of oxygen and argon. The results closely matched REFPROP software calculations, laying foundation for further investigating the influence of impurities on the three-phase equilibrium of fixed points.