Journal of Statistical Physics, Vol. 60, Nos. 3/4, 1990. p. 347-361.
A model is considered in which the bonds of a lattice are covered by rodlike molecules. Neighboring molecular ends interact with orientation-dependent interactions. The model exhibits closed -loop phase diagrams and double critical points. Exact coexistence surfaces are calculated for the model on the Bethe, honeycomb, and square lattices. The nature of the doubling of the critical exponent b near a double critical point is calculated. The behavior of the critical line in the neighborhood of a double critical point is calculated exactly.