Adaptive Accelerated ReaxFF Reactive Dynamics with Validation from Simulating Hydrogen Combustion

Abstract

We develop here the methodology for dramatically accelerating the ReaxFF reactive force field based reactive molecular dynamics (RMD) simulations through use of the bond boost concept (BB), which we validate here for describing hydrogen combustion. The bond order, undercoordination, and overcoordination concepts of ReaxFF ensure that the BB correctly adapts to the instantaneous configurations in the reactive system to automatically identify the reactions appropriate to receive the bond boost. We refer to this as adaptive Accelerated ReaxFF Reactive Dynamics or aARRDyn. To validate the aARRDyn methodology, we determined the detailed sequence of reactions for hydrogen combustion with and without the BB. We validate that the kinetics and reaction mechanisms (that is the detailed sequences of reactive intermediates and their subsequent transformation to others) for H 2 oxidation obtained from aARRDyn agrees well with the brute force reactive molecular dynamics (BF-RMD) at 2498 K. Using aARRDyn, we then extend our simulations to the whole range of combustion temperatures from ignition (798 K) to flame temperature (2998K), and demonstrate that, over this full temperature range, the reaction rates predicted by aARRDyn agree well with the BF-RMD values, extrapolated to lower temperatures. For the aARRDyn simulation at 798 K we find that the time period for half the H 2 to form H 2 O product is ∼538 s, whereas the computational cost was just 1289 ps, a speed increase of ∼0.42 trillion (10 12 ) over BF-RMD. In carrying out these RMD simulations we found that the ReaxFF-COH2008 version of the ReaxFF force field was not accurate for such intermediates as H 3 O. Consequently we reoptimized the fit to a quantum mechanics (QM) level, leading to the ReaxFF−OH2014 force field that was used in the simulations.

High-temperature decomposition of the cellulose molecule: a stochastic molecular dynamics study

“…Thirdly, the reliability of the predictions could be improved by shifting the simulation temperatures towards practical pyrolysis conditions. This could be feasible with recently introduced software and hardware acceleration methods, such as the accelerated ReaxFF reactive dynamics (aARRDyn) (Cheng et al 2014), reactive parallel replica dynamics (RPRD) (Joshi et al 2013) and the GPU-accelerated version of ReaxFF (Zheng et al 2013). Together, the suggested extensions would constitute a ReaxFF-MD framework for predicting the primary reaction pathways of cellulose pyrolysis, the associated kinetics and the main features of the product spectrum.…”

Extracting the mechanisms and kinetic models of complex reactions from atomistic simulation data

“…Similar to the work by Döntgen et al [18] and previous publications, [20,[22][23][24] the chemical species are identified by establishing an atom connectivity map and then using a search algorithm (e.g., depth first) to build molecular fragments. The identified species are represented using the canonical SMILES [35,36] notation, which guarantees the uniqueness of any species labeled.…”

“…Despite the advances in identifying reaction species and rates from simulation data, [12][13][14][15][16][17][18][19][20][21][22][23][24] there is no established method for obtaining reaction mechanisms that can be used directly for engineering (kinetic) modeling. In a recent work by Döntgen et al, [18] a method for identifying reaction species and elementary reactions was proposed.…”

Recent developments of first‐principles force fields

“…The first‐principles methods provide the data for parameter fitting. The Goddard's group has developed a reactive force field (ReaxFF) as shown in Eq. …”

“…A great amount of computational resource is required for the repetitive calculations, especially for crystal structures with large unit cells and low symmetry. Therefore, some works employed the empirical charge methods, such as the charge equilibration (Qeq) method and electron equilibration method to accelerate the charge calculations …”