  A theory of thermotropic nematic liquid crystals in which molecules form
internally ordered clusters is presented. The formulation is based on the same
mean field approximation and form of the anisotropic potential used in the
Maier-Saupe theory. A uniaxial nematic and two macroscopically isotropic phases
are predicted. One of the isotropic phases consists of thermodynamically stable
clusters with internal orientational order. The nematic phase shows cybotactic
order throughout its range of stability. This order persists above the
transition temperature, either as a stable feature or as a pretransitional
effect. The values of the order parameter and of the entropy change at the
nematic to isotropic phase transition depend on the size of the clusters. The
Maier-Saupe theory is obtained in the limit of extremely large or extremely
small clusters.
