Self-consistent tight-binding molecular dynamics simulations of shock-induced reactions in hydrocarbons

Abstract

Abstract.A series of reactive molecular dynamics (MD) simulations of the shock compression of liquid ethane and ethene have been performed using a self-consistent tight-binding (SC-TB) model for hydrocarbons. We employ a recursive purification algorithm for the computation of the density matrix that enables a rapid evaluation of interatomic forces using dense matrix algebra on graphics processing units (GPUs) or sparse matrix algebra for O(N) performance. We achieve a precise long-term conservation of the total energy during microcanonical MD by propagating self-consistently calculated quantities using the extended Lagrangian Born-Oppenheimer MD scheme. No shock-induced reactions were observed during our simulations of liquid ethane, but liquid ethene underwent radical chain polymerization reactions under relatively weak shocks.

Extended Lagrangian Formulation of Charge-Constrained Tight-Binding Molecular Dynamics

“…While this process can lead to a significant reduction in the number of SCF cycles required at each time step before self-consistency is achieved to within a user-defined tolerance, microcanonical trajectories computed using this procedure exhibit a systematic drift in the total (potential plus kinetic) energy. − The magnitude of the drift in the total energy can be reduced, although at increased computation cost, by increasing the number of SCF cycles at each time step. The ability to compute accurate microcanonical trajectories underpins the accuracy of simulations in other ensembles and is pivotal for capturing temperature changes arising from, for example, adiabatic compression or endo- or exothermic chemistry. , …”

Extended Lagrangian Born-Oppenheimer molecular dynamics simulations of the shock-induced chemistry of phenylacetylene

“…[21][22][23] Hugoniostats enable shock-compressed states to be accessed even in small molecular dynamics simulation cells as they avoid the requirement of resolving the propagation of the shock wave directly. However, we have taken a simpler approach 19 since we wish to examine the evolution of a shock compressed-state in the microcanonical, constant NV E, ensemble, that is, in the absence of any coupling to external heat baths, barostats, or hugoniostats.…”

“…V and VI, we use the experimental values 1 for both ρ 0 (0.928 g/cm 3 ) and c 0 (1390 m/s) in Eq. (19) so that we mimic the thermophysical conditions on the experimental non-reacted Hugoniot of liquid phenylactylene.…”

“…19 By simulating dynamic compression explicitly, we have been able to capture adiabatic shock heating directly and the transient, out of equilibrium vibrational populations that are generated immediately behind a propagating shock front. Upon the application of the shrinking cell boundary condition, we monitor the evolution of the temperature, stress tensor, and HOMO-LUMO gap to detect the onset and propagation of any shock induced chemical reactions.…”