Free energy and vapour-liquid equilibria for a quadrupolar Lennard-Jones fluid

Abstract

Monte Carlo (MC) calculations using umbrella sampling are reported for the free energy of a fluid of Lennard-Jones (LJ) molecules having an embedded quadrupole of strength Q*²=Q ²/εσ⁵=2·0. A wide range of temperatures and densities are studied for systems of 32 and 108 molecules, and the vapourliquid coexistence curve is calculated. System size is found to have very little effect, except on the coexistence curve close to the critical point. The MC results are compared with the predictions of the Padé approximant and generalized mean field (GMF—also known as the linearized hypernetted chain) theory. Good agreement is obtained at high temperatures, but the agreement is rather poorer at lower temperatures. The Padé theory is in better overall agreement with the MC calculations than is the GMF theory. The agreement of these theories with MC results is much better than for the dipolar hard sphere system, presumably because of the use of the LJ in place of the hard sphere model.