Molecular theory for fluids modelled with gaussian overlap potentials

Abstract

In recent years it has become evident that molecular shape is an important factor in determining the equation of state and structure of molecular fluids. In formulating a molecular theory of the equation of state it is important to use a model for the molecular interactions which adequately includes the effect of shape but at the same time is computationally simple. The Gaussian overlap potential can be used effectively to model systems of highly non-spherical shape while retaining the simplicity of a single site potential. The model is applicable to both oblate and prolate molecules. We present calculations, using thermodynamic perturbation theory and Monte Carlo simulations, of the thermodynamic properties and structure of fluids of molecules interacting with a Gaussian overlap potential. Both oblate and prolate molecules are considered. The perturbation theory is based on an extension of the Weeks-Chandler-Andersen theory to orientation dependent potentials, and uses a Mayer-function expansion about a spherically averaged reference potential to compute the structure of the hard core reference system. Results are presented for a prolate model with a length to breadth ratio similar to nitrogen and an oblate model with a length to breadth ratio similar to that of benzene. The predictions of the perturbation theory are compared with the computer simulation results. The theory predicts the structural features of the fluids quite accurately except for the most anisotropic system at high density. The thermodynamic properties of the prolate system are given quite accurately by the theory but for the oblate system, which is more anisotropic, the agreement is less satisfactory.