Nonholonomic Hamiltonian method for reacting molecular dynamics
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Macroscale, mesoscale, and ab initio models of reacting shock physics are based, in their most general forms, on rate law descriptions of the chemical processes of interest. Reacting molecular dynamics simulations, by contrast, typically employ potential functions (holonomic Hamiltonian methods) to model chemical reactions. An alternative approach to reacting molecular dynamics models the bonding-debonding process using a rate law, resulting in a nonholonomic Hamiltonian formulation. In previous work at macro and meso scales, discrete nonholonomic Hamiltonian methods have been applied to develop very general models of shock impact and fragmentation process. In this dissertation a similar nonholonomic modeling methodology is used, at the molecular scale, to explicitly model transient chemical processes. Note that the chemistry problem is much more difficult, since both dissociation (fragmentation) and the formation of new molecules must be modeled. The result is the first general reacting molecular dynamics formulation which explicitly models chemical kinetics. Simulation results using this method show good agreement with experiment, for energy release and detonation products in two widely used explosives (HMX and RDX). The reacting molecular dynamics simulation results are used to propose reaction mechanisms and species concentration based kinetics models suitable for use in meso and macro scale shock to detonation simulations. Computational modeling of energetic materials is capable of estimating molecular behavior under conditions not amenable to direct experimental measurement. Further development of RMD methods may help to provide a better understanding of energetic material behavior. This in turn may help to develop improved insensitive high energy density materials.