Reactive Molecular Dynamics Simulation of Fullerene Combustion Synthesis: ReaxFF vs DFTB Potentials

摘要

The dynamic fullerene self-assembly process during benzene combustion was studied using classical Reactive Force Field (ReaxFF) nonequllibrium molecular dynamics (MD) simulations. In order to drive the combustion process, the hydrogen to carbon (H/C) ratio was gradually reduced during the course of the MD simulations. Target temperatures of 2500 and 3000 K were maintained by using a Berendsen thermostat. Simulation conditions and hydrogen removal strategies were chosen to match closely a previous quantum chemical MD (QM/MD) study based on the density-functional tight-binding (DFTB) potential (Saha et al. ACS Nano 2009, 3, 2241) to allow a comparison between the two different potentials. Twenty trajectories were computed at each target temperature, and hydrocarbon cluster size, C_xH_y composition, average carbon cluster curvature, carbon hybridization type, and ring count statistics were recorded as a function of time. Similarly as in the QM/MD simulations, only giant fullerene cages in the range from 155 to 212 carbon atoms self-assembled, and no C_(60) cages were observed. The most notable difference concerned the time required for completing cage self-assembly: Depending on temperature, it takes between 50 and ISO ps in DFTB/MD simulations but never less than 100 ps and frequently several 100s ps in ReaxFF/MD simulations. In the present system, the computational cost of ReaxFF/MD is about 1 order of magnitude lower than that of the corresponding DFTB/MD. Overall, the ReaxFF/MD simulations method paints a qualitatively similar picture of fullerene formation in benzene combustion when compared to direct MD simulations based on the DFTB potential.