Shock-induced melting of (100)-oriented nitromethane: structural relaxation

Area of Science:

• Computational materials science
• Chemical physics
• Shock wave physics

Background:

• Shock wave compression induces significant molecular distortions and large displacements.
• Analyzing molecular dynamics of perturbed materials requires relating time-dependent properties to equilibrium geometry descriptors.
• The Eckart-Sayvetz condition offers a method for aligning molecular coordinate frames for analysis.

Purpose of the Study:

• To apply the Eckart-Sayvetz condition in molecular dynamics simulations of shocked crystalline nitromethane.
• To investigate the response of nitromethane to shock waves using a spatiotemporal reference frame.
• To compute structural and dynamic properties to understand the melting instability.

Main Methods:

• Molecular dynamics simulations of crystalline nitromethane subjected to shock waves (0.5–3.0 km/s).
• Utilized a vibrationally accurate, nonreactive force field.
• Employed a spatiotemporal reference frame centered on the shock front, located via kinetic energy gradients.
• Calculated orientational order parameters, radial distribution functions, and self-diffusion coefficients with 4 fs resolution.

Main Results:

• Clear evidence of melting observed for impact velocities of 2.0 km/s and above.
• Detailed insights into molecular-level changes preceding the melting instability.
• Structural and dynamic properties were computed as the system evolved toward equilibrium.

Conclusions:

• The Eckart-Sayvetz condition effectively facilitates the analysis of molecular dynamics in shocked materials.
• Shock compression of crystalline nitromethane leads to melting at specific impact velocities.
• The study provides a molecular-level understanding of shock-induced melting phenomena.