270 Perturbation Theories for Fluids and Solids
The Parsons–Lee decoupling approximation uses the exact second virial coef-
ficient for the nematic fluid and approximates the higher-order virial coefficients
with those of the HS fluid scaled by B
2
/B
HS
2
. Tjipto-Margo and Evans (1990) deter-
mined the exact third virial coefficient for hard ellipsoids of revolution and used
it, in addition to the second one, to obtain the free energy from Equations 8.155
and 8.156, with the latter truncated at n = 3. They concluded that this approxima-
tion is accurate for predicting the density dependence of the order parameter for
aspect ratio κ ≥ 5, whereas for lower values of κ the influence of higher-order
virial coefficients is not negligible. To incorporate