Structural disorder driven component dependent decomposition pathways and intermolecular interactions in HMX/1,4-PDCA energetic cocrystals: an ab initio molecular dynamics study

Abstract

Structural disorder in energetic cocrystals is recognized to influence their decomposition behavior, yet the underlying trends remain largely unexplored. In this work, the early-stage decomposition pathways and intermolecular interactions of both crystalline and amorphous HMX/1,4-PDCA cocrystals are investigated via ab initio molecular dynamics simulations at 2500 K. Hirshfeld surface analysis reveals that the intermolecular environment in the amorphous model experiences greater strain, characterized by fewer O···H contacts alongside increased O···O and N···O interactions, which ultimately shape the decomposition behavior. In both models, the decomposition of the cocrystal is initiated by the breakdown of HMX; however, the dominant initiation pathway critically depends on molecular packing. Specifically, the crystalline model decomposes via H-dissociation, whereas the amorphous model preferentially undergoes nitro group dissociation. Structural disorder exerts a component-dependent effect, favoring HMX decomposition while simultaneously inducing more complex and heterogeneous degradation behavior in 1,4-PDCA. The amorphous system exhibits higher initial potential energy, faster fragmentation, and enhanced atomic mobility, all pointing to accelerated decomposition kinetics. In contrast, the crystalline model displays lower final potential energy and greater yields of stable products such as H₂O and N₂, suggesting a more thermodynamically complete reaction pathway. Collectively, these findings provide valuable insights into the role of structural disorder in modulating intermolecular interactions and decomposition mechanisms in energetic cocrystals.