Biaxial nematics with C2h symmetry composed of calamitic particles. A   molecular field theory
  A molecular field theory of biaxial nematics formed by molecules with C2h point group symmetry has been developed by Luckhurst et al. and a Monte Carlo computer simulation study of this model has been performed by Hashim et al.. In these studies the truncated model pair potential was only applied to molecules whose long axes are taken to be along their C2 rotation axes. The present study extends this work by assuming that the molecular long axis is now perpendicular to the C2 axis, resulting in there being two possible choices of minor axes. It considers the phases formed by both cases. The molecular field theory for these models is formulated and reported here. The theoretical treatment of the present cases gives rise to a new set of order parameters. So as to simplify the pseudo-potentials only the dominant second rank order parameters are considered and evaluated to give the phase behaviour of these truncated models. The predicted phase behaviour is compared with the results from the molecular field study of the previous model potential. 
Rauzah Hashim, Geoffrey R. Luckhurst and Hock-Seng Nguan
cond-mat.soft