In the mean field approximation, it was assumed that each molecule is subjected to an average internal field which is independent of any local variations or short-range ordering. The orientational energy of a molecule may be postulated as where θ is the angle which the molecular axis makes with the preferred axis, and S is the order parameter. The thermodynamic properties of an ordered system relative to those of the disordered one can be derived on the basis of above formula. According to this relation the variation of the order parameter S versus temperature may be evaluated to get the clear points of liquid crystals under mean field approximation. The Gay-Berne potential was proved to be of considerable value in computer simulation studies of liquid crystals. The potential function g(θ) was tried to form by using Gay-Berne functional. According to our experimental studies for the triphenylene derivatives containing amide groups with the structure of C₁₈H₆(OC₅H₁₁)_6－n(OCH₂CONHC₄H₉)_n (n＝0～3), the more amide groups the molecule contains, the higher the clear point is. These molecules have been studied with an ONIOM (HF/STO-3G: UFF) method. The interaction energies between two molecules in four configurations have been evaluated. Then the parameters in the Gay-Berne potential have been determined by comparison with these interaction energies. The clear point data obtained from the mean field approximation are in agreement with the experimental results. The higher clear points for molecules containing more amide groups are due to forming more intermolecular hydrogen bonds between amide groups.

Key words: discitic liquid crystal, clear point, mean field, Gay-Berne potential, ONIOM method