Square-well diatomics: Bulk equation of state, density profiles near walls, virial coefficients and coexistence properties

Abstract

The thermodynamics of fluids composed of tangent diatomics that interact via site-site square-well potentials is studied. Two different routes to the equation of state are investigated: integral equations and perturbation theory. Reference interaction site model (RISM) theory with the mean spherical approximation (MSA) is used to obtain an equation of state via integral equations. Perturbation theory is based on the hard dimer reference fluid, where the local structure of the reference fluid is obtained from Monte Carlo simulation. The theories are tested against Monte Carlo simulations for the pressure, internal energy, coexistence curve, and the second and third virial coefficients. Monte Carlo simulation data for the pressure are obtained from simulations of the fluid confined between hard walls, where the value of the density at the wall yields the bulk pressure. Second and third virial coefficients are calculated from Monte Carol evaluation of the cluster integrals, and the coexistence curve is obtained from Gibbs ensemble simulations. The theories are in good agreement with the simulation data. The RISM/MSA theory is particularly accurate, and its performance (when compared to simulations) is better than the perturbation theory.