Reactive force fields: Concepts of reaxff and applications to high-energy materials

While quantum-mechanical (QM) methods allow for highly accurate atomistic-scale simulations, their high computational expense limits applications to fairly small systems (generally smaller than 100 atoms) and mostly to statical, rather than dynamical, approaches. Force field (FF) methods are magnitudes faster than QM methods, and as such can be applied to perform nanosecond-dynamics simulations on large (≫1000 atoms) systems. However, these FF methods can usually only describe a material close to its equilibrium state and as such cannot properly simulate bond dissociation and formation. This article describes how the traditional, nonreactive FF concept can be extended in reactive force fields for applications including reactive events by introducing bond order/bond distance concepts. It will discuss how the transferability of the reactive FF can be improved by combining covalent, metallic, and ionic elements. All these concepts will be described by following their implementation in a particular branch of reactive force fields, the ReaxFF reactive force fields, which has found applications to a wide range of materials. Furthermore, we will highlight a series of recent and ongoing applications of ReaxFF force fields to energetic materials, including applications to nitramines, binders, and metallic high-energy materials.