Exploring the High-Pressure Polymorphs of Flexible Molecules by Crystal Structure Prediction: A Case of 1,3,5-Trinitro-1,3,5-Triazinane

Exploring the high-pressure polymorphs of energetic compounds is of great interest in evaluating their performances since they always suffer from high pressure in service. We conducted crystal structure prediction (CSP) of 1,3,5-trinitro-1,3,5-triazinane (RDX) at ambient and high pressures by a combined method to emulate energetic crystals in practical use. That is, we first modified a force field (FF) by considering the flexibility of the RDX molecule to primarily screen out the most energetically favored crystal structures under common conditions, and subsequently, the dispersion-corrected density functional theory (DFT-D) method was applied to refine and rank the structures under common and high-pressure conditions. Thereby, all four polymorphs of RDX with experimentally determined crystal structures are reproduced. Apart from the relatively low ranking of β-RDX, α- (1 at 0 GPa), ε-, and γ-RDX (3 and 1 at 5 GPa, respectively) rank very high in the relative energy landscape. The inferiority of the energy ranking of β-RDX suggests a possible kinetic domination in formation. Moreover, a potential high-pressure form, putative η-RDX, is identified as it ranks 1 and 2 in a pressure range of about 2.3 to 3.5 GPa and at 5 GPa, respectively, and needs further experimental verification. Our work validates the modified FF and DFT-D combined method to CSP, whether under pressure loading or not.