Statistical thermodynamics of long straight rigid rods on triangular   lattices: nematic order and adsorption thermodynamic functions
  The statistical thermodynamics of straight rigid rods of length $k$ on triangular lattices was developed on a generalization in the spirit of the lattice-gas model and the classical Guggenheim-DiMarzio approximation. In this scheme, the Helmholtz free energy and its derivatives were written in terms of the order parameter $\delta$, which characterizes the nematic phase occurring in the system at intermediate densities. Then, using the principle of minimum free energy with $\delta$ as a parameter, the main adsorption properties were calculated. Comparisons with Monte Carlo simulations and experimental data were performed in order to evaluate the reaches and limitations of the theoretical model. 
D. A. Matoz-Fernandez, D. H. Linares and A. J. Ramirez-Pastor
cond-mat.stat-mech physics.chem-ph