Molecular Dynamics Simulations for Describing Chemical Reactions

By definition, “chemical reactions” should involve the breaking and the forming of bonds between atoms and are not usually described by using molecular dynamics (MD) simulations employing classical force fields. Here, the major huddle is that the change in the chemical bonds can only be understood as a pure quantum-mechanical (QM) effect, leading to a change in the fundamental equation of motion for MD from one described by Newtonian mechanics to one described by quantum mechanics. In this article, we introduce recent approaches, reactive force field (ReaxFF) and electron force field (eFF), that were developed to overcome the limitations originating from the extremely heavy computational cost to numerically solve the timedependent QM equation. These new efforts allow largescale MD simulations by solving classical (or classical-like) equations of motions where QM effects are essentially approximated by introducing the concept of bond order (ReaxFF) or by describing the electrons with breathing Gaussian wavepackets (eFF). We then discuss how these new simulation methodologies can be applied to investigate the complicated dynamical problems in which chemical reactions and/or phase transitions induced by electronic excitations are strongly involved.