Molecular field theory of nematics: density functional approach. I. Bulk effects

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Abstract
The molecular field theory of nematics is rephrased in density functional language. The free energy in the nematic state is expressed as a functional expansion around the isotropic state in powers of the nematic order parameters Pl and the fractional density change eta at the isotropic-nematic transition. The crucial quantities in the expansion are spherical harmonics of the direct correlation functions in the isotropic state. Sensible approximations to these direct correlation functions lead to other well known formulations of nematic field theory. An exact (in the authors' approximation) result for eta is obtained in terms of the potential parameters Ul and the order parameters Pl at the phase transition. The authors discuss the relationship of their approach to other microscopic theories of nematics. The theory can be extended to discuss inhomogeneous systems.