Multiscale AIMD-RMD study of acancy- and void-controlled hotspot formation in RDX

High-energy-density materials (HEDMs) are widely used in explosives and propellants, but their sensitivity to shock, temperature, and defects remains a critical limitation for both safety and combustion performance. Although previous studies have identified decomposition pathways and developed macroscopic ignition models, the direct link between microscopic-scale defects and reaction kinetics under shock loading remains insufficiently understood. In this work, we develop a multiscale computational framework to determine how molecular vacancies influence the decomposition mechanisms and shock response of 1,3,5-trinitro-1,3,5-triazine (RDX). By combining ab initio molecular dynamics (AIMD) with large-scale reactive molecular dynamics (RMD), we quantitatively assess how molecular vacancies, located in the unit cells surrounding nanoscale void, influence chemical reaction kinetics, hotspot formation, and the coupled thermomechanical response over a broad range of temperatures and shock velocities. AIMD results reveal that vacancy-containing cells undergo significantly accelerated reactions, exhibiting lower activation energies than vacancy-free cells (47.23 kcal·mol⁻¹ vs. 51.14 kcal·mol⁻¹). At 1100 K, the characteristic reaction time decreases from 196 ns in vacancy-free cells to 65 ns in vacancy-containing cells. RMD shock simulations reveal a pressure-dependent, dual-mechanism behavior: at lower particle velocities (2.0∼2.5 km·s⁻¹) vacancies enhance local thermalization and accelerate hotspot growth, whereas above a critical velocity (3.0 km·s⁻¹) shock pressure and pressure-volume ( p -V) work from void collapse dominate hotspot evolution and mask vacancy effects. Overall, these results provide a quantitative, physics-based framework for predicting ignition thresholds associated with void defects and establish a microscopic foundation for next-generation detonation models.