No abstract

An analysis of the known approximations for describing the dependence of the heat capacity at a constant volume of energetic materials (molecular crystals) on the crystal temperature has shown that there are reliable approximations of the dependence of the heat capacity at a constant volume that do not require complex quantum mechanical calculations to determine the frequencies of normal vibrations, both intermolecular and inside the molecule. To obtain the dependence of the thermal part of the internal energy of a molecular crystal, which is responsible for heating the material, it is required to integrate the heat capacity expression at constant volume over temperature. In this work, calculations have been made for the dependence of the thermal part of the internal energy of a molecular crystal in case when it is calculated through the frequencies of normal vibrations, and in case when it is calculated by integrating the heat capacity at a constant volume with respect to temperature using approximation formulas. When solving the spectral problem of determining the frequencies of normal vibrations within the molecule, the PM3 and DFT quantum chemical methods have been used. The paper presents the dependences of the thermal part of the internal energy of molecular crystals on temperature, calculated for different methods of determination, and a comparative analysis, which has shown that the difference has equaled less than 1 %.