Molecular dynamics simulations of void defects in the energetic material HMX

Area of Science:

• Materials Science
• Computational Chemistry
• Chemical Engineering

Background:

• Crystalline energetic materials like octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) contain defects that influence their stability and performance.
• Understanding the behavior of voids, a common type of defect, is crucial for predicting material properties and ensuring safety.

Purpose of the Study:

• To investigate the dynamic evolution and energetic properties of void defects in crystalline HMX using molecular dynamics (MD) simulations.
• To explore the impact of void size on defect stability, molecular behavior, and conformational changes within the HMX lattice.

Main Methods:

• Construction of various HMX models with a consistent vacancy concentration (10%) to study void size effects.
• Employing annealing simulations to determine energetic ground state properties, including void formation and binding energies.
• Conducting 1 nanosecond (ns) MD simulations to analyze molecular dynamics, mean square displacements (MSDs), and conformational changes.

Main Results:

• Void formation energy decreases with increasing void size, ranging from 55-63 kcal/mol(-1).
• Average binding energy per molecule ranges from 32-34 kcal/mol(-1), decreasing for molecules near the void surface.
• Larger voids demonstrate increased susceptibility to collapse, with collapsed HMX molecules adopting liquid-like structural characteristics.
• Four unique low-energy HMX conformers were identified within voids, including two not observed in crystalline phases, with their ratios varying by temperature.

Conclusions:

• Void size significantly influences the stability and collapse dynamics of defects in crystalline HMX.
• The presence of voids induces unique molecular conformations and liquid-like behavior in HMX, deviating from crystalline structures.
• Temperature plays a role in the conformational landscape of HMX molecules within voids, favoring specific conformers at higher temperatures.