Effect of molecular elongation on the quadrupolar free energy in diatomic fluids

Abstract

This paper reports results obtained from computer simulations of pure fluids composed of two-centre Lennard-Jones plus point quadrupole molecules. Simulations were performed over a range of quadrupole moments 0 ⩽ Q ² ⩽ 4 and a range of molecular elongations 0 ⩽ l ⩽ 0·793. The principal results reported are values of the quadrupolar contribution to the Helmholtz free energy A_Q , which was computed via Kirkwood's coupling parameter method. We show how A_Q responds to changes in both Q and l. Results are also presented for the configurational internal energy, the quadrupolar internal energy, and site-site pair distribution functions. The simulation results are used to test two forms of first-order perturbation theory; neither form of the theory satisfactorily predicts the simulation results when both the quadrupole moment and the molecular elongation are large.