Thermodynamic perturbation theory coefficients for ellipsoidal molecules

Abstract Perturbation theory has been the center of the development of the new generation of equations of state. First- and second-order perturbation theories are very common, but require approximations for obtaining an analytical form. Recently, a new equation of state has been proposed in which the reference fluid is based on the hard Gaussian overlap approach, and the perturbed potential is defined as a spherically symmetric square well. In such an approach, the first- and second-order coefficients were considered the same as the ones applied for a system in which the reference term is spherical. Using Monte Carlo simulations, we investigated the validity of such an approximation by calculating the first- and second-order coefficients of the high-temperature expansion series of the Helmholtz free energy. With our findings, this approximation seems to be quite reasonable for a certain range of anisotropies. We also present a calculation of the perturbed molar Helmholtz free energy using Monte Carlo simulations, which could in principle be used for improving the equation of state.