Electronic structure and molecular dynamics of breaking the RO–NO2 bond

Investigation of nonadiabatic dynamics in the photolysis of methyl nitrate (CH3ONO2) by on-the-fly surface hopping simulation

The photolysis mechanism of methyl nitrate (CH₃ONO₂) was studied using the on-the-fly surface hopping dynamics at the XMS-CASPT2 level. Several critical geometries, including electronic state minima and conical intersections, were obtained, which play essential roles in the nonadiabatic dynamics of CH₃ONO₂. The ultrafast nonadiabatic decay dynamics to the ground state were simulated, which gives a proper explanation on the broad and structureless absorption spectra of CH₃ONO₂. The photodissociation channels, including CH₃O + NO₂, CH₃O + NO + O, and others, as well as their branching ratios, were identified. When the dynamics starts from the lowest two electronic states (S₁ and S₂), the CH₃O + NO₂ channel is the dominant photolysis pathway, although we observed the minor contributions of other channels. In contrast, when the trajectories start from the third excited state S₃, both CH₃O + NO₂ and CH₃O + NO + O channels become important. Here the CH₃O-NO₂ bond dissociation takes place first, and then for some trajectories, the N-O bond of the NO₂ part breaks successively. The quasi-degeneracy of electronic states may exist in the dissociation limits of both CH₃O + NO₂ and CH₃O + NO + O channels. The current work provides valuable information in the understanding of experimental findings of the wavelength-dependent photolysis mechanism of CH₃ONO₂.

Car-Parrinello Monitor for More Robust Born-Oppenheimer Molecular Dynamics

Born-Oppenheimer molecular dynamics (BOMD) is a promising simulation method for exploring the possible reaction pathways of a chemical system, but one significant challenge is the increased difficulty of converging the self-consistent field (SCF) calculation that often accompanies the breaking and forming of chemical bonds. To address this challenge, we developed an enhancement to the BOMD simulation method called the Car-Parrinello monitor (CPMonitor) that uses Car-Parrinello molecular dynamics (CPMD) to recover from SCF convergence failures. CPMonitor works by detecting SCF convergence failures in BOMD and switching to a CPMD Hamiltonian to propagate through the region of configuration space where the SCF calculation is unable to converge, then switching back to BOMD when good convergence behavior is re-established. We present a series of simulation studies that use CPMonitor, including detailed studies of the thermodynamic and dynamical properties of simple systems, as well as ab initio nanoreactor simulations containing transition metal atoms that were previously not possible to simulate using standard BOMD methods. Our studies show that CPMonitor can make BOMD simulations robust to SCF convergence difficulties and improve simulation performance and stability in reaction discovery applications.

Molecular dynamics simulations of shock waves in hydroxyl-terminated polybutadiene melts: mechanical and structural responses

The mechanical and structural responses of hydroxyl-terminated cis-1,4-polybutadiene melts to shock waves were investigated by means of all-atom non-reactive molecular dynamics simulations. The simulations were performed using the OPLS-AA force field but with the standard 12-6 Lennard-Jones potential replaced by the Buckingham exponential-6 potential to better represent the interactions at high compression. Monodisperse systems containing 64, 128, and 256 backbone carbon atoms were studied. Supported shock waves were generated by impacting the samples onto stationary pistons at impact velocities of 1.0, 1.5, 2.0, and 2.5 km s(-1), yielding shock pressures between approximately 2.8 GPa and 12.5 GPa. Single-molecule structural properties (squared radii of gyration, asphericity parameters, and orientational order parameters) and mechanical properties (density, shock pressure, shock temperature, and shear stress) were analyzed using a geometric binning scheme to obtain spatio-temporal resolution in the reference frame centered on the shock front. Our results indicate that while shear stress behind the shock front is relieved on a ∼0.5 ps time scale, a shock-induced transition to a glass-like state occurs with a concomitant increase of structural relaxation times by several orders of magnitude.