Surface Defect Formation Mechanism of ε‑2,4,6,8,10,12-Hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20) and Solvent Effects by Biased Molecular Dynamics Simulations

Surface defects can significantly affect the performances\nof energetic crystals, particularly safety and mechanics, and deepening\nthe insight of the surface defect formation mechanism is expected\nto facilitate the defect engineering. Nevertheless, the mechanism\ngenerally remains unclear, primarily due to a long time span of the\nformation, making it difficult to capture it by conventional molecular\nsimulations. To address this, this work takes the most powerful energetic\ncompound commercialized already, 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane\n(CL-20), as a sample; combines quantum chemistry and machine learning\nmethods to refine a force field for it, the OPLS-CL-20 force field;\nand performs well-tempered metadynamics simulations on the main crystal\nfaces of ε-CL-20 in six solvents to explore the vacancy defect\nformation mechanism thereby. It is found that, among all the main\ncrystal faces, (002) exhibits the largest molecular detachment work\nacross all solvents and preserves the highest perfection, and all\nfaces show greater molecular detachment work in the DMF solution relative\nto others, implying the highest possibility of ε-CL-20 keeping\nperfect therein. Our study may provide theoretical guidance for the\nsolvent selection and crystal quality control of molecular crystalline\nmaterials.