We study the effects of an external field on the isotropic-nematic transition in liquid-crystalline substances. The theoretical technique employed is called the "orientationally averaged pair correlations" approximation. It takes into detailed account spatial correlations between the molecules while treating orientational order in a mean-field-like manner. The results obtained for MBBA ($4-\mathrm{m}\mathrm{e}\mathrm{t}\mathrm{h}\mathrm{o}\mathrm{x}\mathrm{y}\mathrm{b}\mathrm{e}\mathrm{n}\mathrm{z}\mathrm{y}\mathrm{l}\mathrm{i}\mathrm{d}\mathrm{e}\mathrm{n}\mathrm{e}-4\prime -n-\mathrm{b}\mathrm{u}\mathrm{t}\mathrm{y}\mathrm{l}\mathrm{a}\mathrm{n}\mathrm{i}\mathrm{l}\mathrm{i}\mathrm{n}\mathrm{e}$) are compared to those of the Maier-Saupe theory and the Landau-de Gennes theory. We determine the paranematic-nematic coexistence curve, the temperature-external field phase diagram, the Cotton-Mouton coefficient, the maximum supercooling temperature, critical exponents, and the relation between the transition temperature and the electric field in laser-induced isotropic-nematic transitions. These results are compared to experimental data. Their implications are discussed.

• Received 13 January 1981

DOI:https://doi.org/10.1103/PhysRevA.24.493