Fluid properties from equations of state compared with direct molecular simulations for the Lennard-Jones system

Abstract

Evaluation of the fluid properties obtained from thermodynamic derivatives is a critical test for any equation of state, especially in the vicinity of the critical point. This is a state region of great importance for the nonclassical gas dynamics of dense vapors with unconventional dynamic phenomena. In this study, the isobaric and isochoric heat capacity, coefficient of thermal expansion, isothermal compressibility, speed of sound and the Joule-Thomson coefficient have been calculated from three accurate multi-parameter equations of state for a Lennard-Jones fluid. The results are compared with direct molecular simulations in a NVEPG ensemble in which thermodynamic derivatives for arbitrary order can be determined directly without any restrictive approximations. Here, a direct comparison of thermodynamic derivatives obtained from an equation of state and precisely calculated values is possible. It is shown that all three equations of state evaluate fluid properties well for state points away from the critical point, whereas the quality in the vicinity of the critical point can be very poor.