Solid–solid polymorphic transition of 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20) <b>at high temperature</b> by metadynamics simulations

It remains challenging to reveal the solid-solid polymorphic transition mechanism of flexible molecular crystals as it involves complex molecular conformational changes and packing rearrangements. Taking 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20), a molecule with flexible -NO₂ rotations and a relatively large molecular size, as an example, this work investigates its polymorphic behavior at high temperature. A refined force field, OPLS-CL-20, specifically developed to accurately capture both the energetic and geometric features of CL-20 molecular conformations and lattice structures, is employed. Combined with the well-tempered metadynamics method, this approach successfully simulates the solid-solid polymorphic transitions among the ε-, β-, and γ-forms, which are all stable under ambient conditions. Therein, all the forms are reproduced, and transition state structures are verified. The analysis of relative energy levels based on the free energy landscape for polymorphic transitions confirms a stability decreasing order of ε-, γ- and β-CL-20. Meanwhile, cracking occurs only in heated ε-CL-20, due to the highest energy deposition producing the strongest stress. In addition, the step-by-step and concerted molecular conformation variation patterns of neighboring molecules are found during the polymorphic transition. The facile polymorphic transition of CL-20 is attributed to the easy -NO₂ rotation and high similarity in the lattice structures of the three polymorphs. Hopefully, this work will deepen the understanding of the solid-solid polymorphic transition mechanism of molecular crystalline materials.