Thermodynamic properties of nonpolar molecular fluids of different geometries from perturbation theory

Abstract

Advances in statistical mechanics during the 1960s and 1970s allow us to determine with great accuracy thermodynamic properties of simple 1iquids.lJ Thus, for noble gases, Malijevsky et ala3 have shown that it is possible to determine their thermodynamic behavior from theoretical grounds. The high accuracy of the results makes them useful for practical applications. For molecular liquids, important progress has been made during the last decade, especially on integral equations and perturbation theories although the quality of the results is not comparable to that of simple liquids yet.4 Perturbation the0ryl3~J appears as the best candidate for practical applications since it is both accurate and fast from a computational point of view. Accurate perturbation theories are now available for several kinds of molecular fluids such as spherical or quasi-spherical polar fluids4v6 and for anisotropic nonpolar fluids interacting through a site-site potential The site-site potential model presents, however, the important disadvantage of a quadratic increase of the number of interactions between a pair of molecules as the number of sites per molecule grows. Therefore, the computational time required for calculations increases quadratically with the molecular complexity and that prevents an easy extension of the theories to complex molecules. In 1951, Kihara'O proposed a pair potential which only depends on the shortest distance p between the molecular cores. The cores are chosen to reproduce the molecular shape. Thus, in this model the pair interaction is described by only one interaction regardless of the molecular complexity. Although the time required for the evaluation of p increases slightly with the molecular complexity this increase is far from being quadratic. Hence, the interest of using Kihara potential as an effective pair potential for complex molecules. The simulation studies performed for the Kihara model in our laboratory,l'-I3 either by Monte Carlo (MC) or by molecular dynamics (MD), have shown that it is easy to carry out simulations of these models and that Kihara potential constitutes a good effective pair potential for the liquid phase. The obtained results were not worse than those obtained with the more popular site-site model. Simulation studies yield exact results for the (I) Hansen, J. P.; McDonald, I. R. Theory ofSimple Liquids; Academic (2) Boublik, T.; Nezbeda, I.; Hlavaty, K. Statistical Thermodynamics of