We present preliminary results on the structural and mechanical properties of PETN and TATB as obtained by ab-initio calculations. While first principles approaches describe properties of materials occurring at 0 K, experimental results are mainly provided at ambient temperature. This difference leads to discrepancies in the desired agreement of calculated and measured properties. Also, the congenital limitation of DFT in dealing with long-range dispersion interactions, provide intrinsic limitations in this attempt. Our results are obtained through a full 3D approach that takes into account the van der Waals interaction in the form of London dispersion forces between the molecules assembled in the solid using the B3LYP-D* functional. Results are compared with available experiments and previous simulation data, showing very good agreement and validating the importance of including volume expansion effects in the description of molecular crystals at temperatures above absolute zero. To the authors' knowledge, the results presented for the elastic constants of TATB are the first ab-initio data for this energetic material.

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