Ac. Branka et Dm. Heyes, THERMODYNAMIC PROPERTIES OF PLANAR SQUARE-WELL DUMBBELL FLUIDS - MONTE-CARLO SIMULATIONS AND PERTURBATION-THEORY, The Journal of chemical physics, 99(12), 1993, pp. 9882-9889
We calculate for the first time the equation of state and other thermo
dynamic properties of the planar square-well dumbbell (SWD) fluid. The
SWD molecule consists of two fused hard discs of diameter sigma, sepa
rated by a reduced separation L = L/sigma with at tractive annuli of
outer radii, Ksigma around each hard disc. Monte Carlo simulations wer
e carried out over a range of densities and temperatures for two choic
es of potential width and several molecular elongations. In particular
, we concentrated on states with the molecular elongation parameter, L
= 0.3 and the potential-width parameter, K = 1.5. In this article we
concentrate on analytic representations of the thermodynamic behavior
of the SWD fluid. The second virial coefficients of the SWD system ar
e calculated by a part analytic/part numerical procedure. The free ene
rgy expansion coefficients are calculated up to third order. Convergen
ce of the perturbation expansion based on the hard-dumbbell fluid is e
xamined and shown to be very satisfactory. A generalization of the qua
sichemical (QCA) coordination number model for the SWD fluid is propos
ed. The equation of state and energy derived from this model, compared
with the simulation results, show that the QCA model represents well
the simulated properties of the SWD at low density but fails at high d
ensity.