hard bar model;  sequence parameters;  Group expansion;  rejection volume;  Free energy

Nematic phase liquid crystals are widely used in TN, STN and other display technologies, and the study of nematic-isotropic phase transition temperature TNI has important theoretical and application significance. Based on L. Onsager’s hard rod theory, the mutual attraction potential between molecules is considered, and an improved model is given. Using the "group unfolding" method, the free energy of the system is derived; Using the Zwanzig method, the self-consistent equation of the order parameters, the equation of state of the system and the resulting transition temperature TNI are closer to the experimental results than the L. Onsager hard bar theory.