Shock-induced detonation mechanism of NH3OH+N5-: a deep potential molecular dynamics study with and without nuclear quantum effect.

CONTEXT: As a novel type of high-energy-density, environmentally friendly, and low-sensitivity energetic materials (EMs), cyclo-pentazolate salts are being extensively studied. However, their detonation mechanism remains unclear. This study developed a neural network potential (NNP) to simulate the shock-induced detonation process of NH3OH+N5-, a representative salt of the pentazolate anion (N5-). The well-trained NNP exhibits high precision comparable to DFT, as well as high efficiency. The NNP-based large-scale molecular dynamics (MD) simulations for NH3OH+N5- produced an ideal C-J detonation velocity of 9.4 km/s, which is in agreement with the value estimated by the Cheetah 7.0 program (9.93 km/s). The simulation demonstrates that the proton transfer from NH3OH+ to N5- is the initial reaction, while the primary decomposition pathway of N5- is a ring-opening reaction, or the bimolecular reactions with its initial decomposition intermediate azide anion N3- resulting in the formation of N8 ring. Quantum chemical calculations show that these pathways possess low activation barriers. The influence of nuclear quantum effects on shock-induced chemical reactions was also studied, which shows that nuclear quantum corrections not only improve the accuracy of predicted ideal detonation velocity but also improve temperature in simulations, which results in the different reaction mechanism of shock-induced detonation reaction of NH3OH+N5-, facilitating the ring-opening reaction of N5- ring and preventing its reaction with N3. This study enhances the understanding of the detonation mechanism of cyclo-pentazolate salts.

METHODS

In this work, NNP potential was trained by the DeePMD-kit package and homemade FORTRAN code. The density functional theory (DFT) calculation of structural energies and atomic forces, as well as ab initio molecular dynamics (AIMD), was conducted using the Vienna Ab initio Simulation Package (VASP) software. The PAW method and the GGA-PBE functional were adopted. The shock wave response MD simulations were conducted by LAMMPS with the multiscale shock technique (MSST) and QB-MSST methods. Quantum chemical calculations were carried out at the M06-2X/TZVP level using the Gaussian 09 program.