The Pople and Karasz theory of melting of molecular crystals was modified using the third repulsive energy parameter in our previous paper. The new modified model is applied to study thermodynamics of solid-nematic and nematic-isotropic liquid transitions. The thermodynamic properties are evaluated by the Bragg-Williams approximation. The results are compared with the predictions of the original Pople and Karasz theory and its volume dependence modification, as well as some experimental data.