Reactive molecular dynamics of hypervelocity collisions of PETN molecules.

Born-Oppenheimer direct dynamics classical trajectory simulations of bimolecular collisions of PETN molecules have been performed to investigate the fundamental mechanisms of hypervelocity chemistry relevant to initiating reactions immediately behind the shock wavefront in energetic molecular crystals. The solid-state environment specifies the initial orientations of colliding molecules. The threshold velocities for initiating chemistry for a variety of crystallographic orientations were correlated with available experimental data on anisotropic shock sensitivity of PETN. Collisions normal to the planes (001) and (110) were found to be most sensitive with threshold velocities on the order of characteristic particle velocities in detonating PETN. The production of NO2 is the dominant reaction pathway in most of the reactive cases. The simulations show that the reactive chemistry, driven by dynamics rather than temperature during hypervelocity collisions, can occur at a very short time scale (10(-13) s) under highly nonequilibrium conditions.

Density functional theory calculations of solid nitromethane under hydrostatic and uniaxial compressions with empirical van der Waals correction.

First-principles density functional theory calculations have been performed with and without an empirical van der Waals (vdW) correction to obtain constitutive relationships of solid nitromethane under hydrostatic and uniaxial compressions. The unit-cell parameters at zero pressure and the hydrostatic equation of state at 0 K are in reasonable agreement with experimental data using pure DFT, and the agreement is significantly improved with the inclusion of the vdW dispersion correction. Uniaxial compressions normal to the {100}, {010}, {001}, {110}, {101}, {011}, and {111} planes were performed, and a comparison of the principal stresses, changes in energy, and shear stresses for different compression directions clearly indicate anisotropic behavior of solid nitromethane upon compression. The calculated anisotropic constitutive relationships might help to link the anisotropic shock sensitivity and the underlying atomic-scale properties of solid nitromethane.

Rectification mechanism in diblock oligomer molecular diodes.

We investigated a mechanism of rectification in diblock oligomer diode molecules that have recently been synthesized and showed a pronounced asymmetry in the measured I-V spectrum. The observed rectification effect is due to the resonant nature of electron transfer in the system and the localization properties of bound state wave functions of resonant states of the tunneling electron interacting with an asymmetric molecule in an electric field. The asymmetry of the tunneling wave function is enhanced or weakened depending on the polarity of the applied bias. The conceptually new theoretical approach, the Green's function theory of sub-barrier scattering, is able to provide a physically transparent explanation of this rectification effect based on the concept of the bound state spectrum of a tunneling electron. The theory predicts the characteristic features of the I-V spectrum in qualitative agreement with experiment.