David Furman

Decomposition of Condensed Phase Energetic Materials: Interplay between Uni- and Bimolecular Mechanisms

Activation energy for the decomposition of explosives is a crucial parameter of performance. The dramatic suppression of activation energy in condensed phase decomposition of nitroaromatic explosives has been an unresolved issue for over a decade. We rationalize the reduction in activation energy as a result of a mechanistic change from unimolecular decomposition in the gas phase to a series of radical bimolecular reactions in the condensed phase. This is in contrast to other classes of explosives, such as nitramines and nitrate esters, whose decomposition proceeds via unimolecular reactions both in the gas and in the condensed phase. The thermal decomposition of a model nitroaromatic explosive, 2,4,6-trinitrotoluene (TNT), is presented as a prime example. Electronic structure and reactive molecular dynamics (ReaxFF-lg) calculations enable to directly probe the condensed phase chemistry under extreme conditions of temperature and pressure, identifying the key bimolecular radical reactions responsible for the low activation route. This study elucidates the origin of the difference between the activation energies in the gas phase (~62 kcal/mol) and the condensed phase (~35 kcal/mol) of TNT and identifies the corresponding universal principle. On the basis of these findings, the different reactivities of nitro-based organic explosives are rationalized as an interplay between uni- and bimolecular processes.

Enhanced Particle Swarm Optimization Algorithm: Efficient Training of ReaxFF Reactive Force Fields

Particle swarm optimization (PSO) is a powerful metaheuristic population-based global optimization algorithm. However, when it is applied to nonseparable objective functions, its performance on multimodal landscapes is significantly degraded. Here we show that a significant improvement in the search quality and efficiency on multimodal functions can be achieved by enhancing the basic rotation-invariant PSO algorithm with isotropic Gaussian mutation operators. The new algorithm demonstrates superior performance across several nonlinear, multimodal benchmark functions compared with the rotation-invariant PSO algorithm and the well-established simulated annealing and sequential one-parameter parabolic interpolation methods. A search for the optimal set of parameters for the dispersion interaction model in the ReaxFF- lg reactive force field was carried out with respect to accurate DFT-TS calculations. The resulting optimized force field accurately describes the equations of state of several high-energy molecular crystals where such interactions are of crucial importance. The improved algorithm also presents better performance compared to a genetic algorithm optimization method in the optimization of the parameters of a ReaxFF- lg correction model. The computational framework is implemented in a stand-alone C++ code that allows the straightforward development of ReaxFF reactive force fields.

Cavitation-Induced Synthesis of Biogenic Molecules on Primordial Earth

Despite decades of research, how life began on Earth remains one of the most challenging scientific conundrums facing modern science. It is agreed that the first step was synthesis of organic compounds essential to obtain amino acids and their polymers. Several possible scenarios that could accomplish this step, using simple inorganic molecules, have been suggested and studied over the years. The present study examines, using atomistic reactive molecular dynamics simulations, the long-standing suggestion that natural cavitation in primordial oceans was a dominant mechanism of organic molecule synthesis. The simulations allow, for the first time, direct observation of the rich and complex sonochemistry occurring inside a collapsing bubble filled with water and dissolved gases of the early atmosphere. The simulation results suggest that dissolved CH4 is the most efficient carbon source to produce amino acids, while CO and CO2 lead to amino acid synthesis with lower yields. The efficiency of amino acid synthesis also depends on the nitrogen source used (i.e., N2, NH3) and on the presence of HCN. Moreover, cavitation may have contributed to the increase in concentration of NH3 in primordial oceans and to the production and liberation of molecular O2 into the early atmosphere. Overall, the picture that emerges from the simulations indicates that collapsing bubbles may have served as natural bioreactors in primordial oceans, producing the basic chemical ingredients required for the beginning of life.