A Monte Carlo study of mixtures of hard diatomic molecules

Abstract

The Monte Carlo method has been used to study a model system of equimolar mixtures of 256 hard diatomic molecules, each consisting of two fused hard spheres having the same diameter σ with the elongations of the components l_a * and l_b * (l*=l/σ). Mixtures with l_a */l_b *=0·2/0·4, and 0·2/0·6 were studied at densities typical of the liquid state. The orientational structure of dense hard diatomic fluids has been studied by calculating up to 13 terms in the expansion of the total pair distribution function g(r ₁₂θ₁θ₂) in spherical harmonics. The coefficients gll'm^ij (r ₁₂) of the series have been calculated as ensemble averages in the simulation. At short distances the system exhibits a high degree of angular correlation, which increases with density and the elongation of the longer component, the correlation is relatively short-ranged, with no significant angular structure beyond twice the major diameter of the larger component. A zero-order blip function perturbation theory is presented for mixtures of non-spherical molecules, and the liquid structure predicted by this theory is compared with the simulation results. The virial pressure of the mixtures is also calculated as a function of the density.