Reactive Molecular Dynamics Methods
ACT has developed a reactive MD based computational framework to investigate chemical behavior of polymer composite materials subject to thermo-chemical degradation in extreme environments. The approach makes use of reactive MD simulations and novel reactive force fields to obtain information on chemical products, rate of formation of these products and reaction pathways. The framework has been developed and applied to study pyrolysis (thermal deposition) phenomena for two types of materials: (i) ablative phenolic heat-shield composite used in space-craft reentry vehicles, and (ii) ablative rubber insulator of solid rocket motor type engines. ACT has developed multiple post-processing tools (MolfrACTTM and KinACTTM) which enable extraction and visualization of chemical reactions and chemical kinetics data in multi-specie complex environments possible.
Potential applications of ACT’s reactive MD computational framework include:
- Design and evaluation of ablative materials
- Design and development of energetic materials
- Design of hydrogen storage materials
- Investigation petrochemical manufacturing methods
Reactive molecular dynamics methods refer to a class of MD methods which enable simulation of chemical reactions and chemistry based nanoscale phenomena with accuracy to ab-initio methods. While classical MD provides a powerful tool to simulate nano-scale spatio-temporal phenomena and bulk properties, they lack the ability to simulate chemical reactions. On the other hand, quantum mechanics (QM) based ab-initio methods enable simulations of chemical reactions, but are computationally expensive and limited to a 10’s of atoms. To circumvent this problem, reactive MD methods have been developed which use empirical interatomic potentials with the capability to locally mimic the quantum effects of a bond change due to chemical reactions.
Reactive MD methods use a force field description which is ‘trained’ to reproduce reactions determined from quantum mechanical calculations and experimental data. A class of such methods uses concept of bond-order to represent the forces between interacting atoms in a chemical system. Typical examples of such force descriptions include: Reactive Empirical Bond-Order potential (REBO), Adaptive Intermolecular Reactive Empirical Bond-Order potential (AIREBO), Reactive Force Field (ReaxFF) potential. The bond order based definition of atomic and molecular interactions excludes the need for predefined reactive sites and allows for dynamic description of bond breaking and formation. Recent progress in reactive potentials enables study of wide range of materials such as hydrocarbons, polymers, silica systems, nitramines and catalysis phenomenon.
Conventional reactive MD methods are limited to high temperature simulations and suffer from small timestep problems inherent to MD (due to stiffness of evolution equations). Simulation temperatures of reactive MD studies are usually performed at temperatures of ~3000K with timesteps on the order of 0.25 femtoseconds. Consequently, simulations can probe 100’s of picosecond of dynamics to a maximum of a nanosecond. However, most interesting phenomena at nanoscale evolve over longer times and sometimes requires lower temperature studies. To address these challenges, ACT has recently developed Accelerated Reactive Molecular Dynamics Simulator (ARMS), a computational tool which can be used to investigate material chemistry at low temperatures using reactive molecular dynamics simulation. The framework makes use of multi-core processors to track many replicas of the system in parallel, effectively parallelizing the time required to simulate chemical reactions at low temperature. The methodology has been used to probe low temperature (of ~1750K) pyrolysis chemistry of ablative heat-shield materials typical to space-craft re-entry vehicles and solid rocket motor insulation. We find that the chemistry at low-temperature is significantly different from high temperatures and results in distinct reaction pathways.