First-principles study on energetic cocrystals of CL-20/4,5-MDNI with two different stoichiometric ratios under high pressure.

Context: This research determined the crystal structure, molecular structure, electronic structure, optical properties, mechanical properties, and Hirshfeld analysis of the CL-20/4,5-MDNI cocrystal at two distinct stoichiometric ratios under hydrostatic pressures varying from 0 to 100 GPa. The findings revealed that the CL-20/4,5-MDNI cocrystal with a 1:1 ratio experienced two structural transitions at pressures of 80 GPa and 90 GPa. Notably, new covalent bonds, C10-O13 and C9-O14, were established, whereas the C10-H10C bond was disrupted. In contrast, the CL-20/4,5-MDNI cocrystal with a 1:3 ratio underwent three structural transformations at pressures of 55 GPa, 63 GPa, and 95 GPa, leading to the creation of new covalent bonds such as C17-N35, C25-N43, C14-O9, C21-O7, and N27-H9. These transitions were corroborated through the examination of lattice parameters, variations in covalent bond lengths, density of states, and optical coefficients. Additionally, the study explored the similarities and differences between the two cocrystals in terms of their crystal structure, molecular structure, electronic properties, optical properties, mechanical properties, and Hirshfeld analysis.

Method: In this investigation, the CASTEP module from the Materials Studio software package was utilized to perform first-principles calculations based on density functional theory (DFT). Specifically, the Broyden-Fletcher-Goldfarb-Shanno (BFGS) optimization technique was applied to refine the geometric structures of the CL-20/4,5-MDNI cocrystals, which were prepared in the stoichiometric ratios of 1:1 and 1:3. These calculations were conducted under a range of hydrostatic pressures, varying from 0 to 100 GPa. To achieve a fully relaxed state at atmospheric pressure, the Perdew-Zunger local density approximation (LDA/CA-PZ) functional was employed. The plane wave cutoff energy was meticulously set at 489 eV to ensure the convergence of the total energy within the unit cell system. Additionally, the k-point mesh was configured as 1 × 1 × 1 to facilitate accurate calculations. Before each simulation, different hydrostatic pressures were systematically applied to analyze the structural changes under varying conditions.