A Reactive Explicit Electron Force Field for Hydrocarbons.

The prevailing reactive force fields are typically based on the bond-order concept or other predefined chemical frameworks, along with implicit electronic treatments and the incorporation of intricate corrections, which severely limit their further development. Explicit electron force fields introduce electrons as particles into the force field, and their ability to model electronically excited systems as well as electronic response properties has been demonstrated, yet the description of ground-state reactions for complex systems remains challenging. This study introduces a new explicit electron force field (EeFF) for modeling hydrocarbon ground-state reactions. The EeFF explicitly treats electrons as distinct wave packets with variable radii and variable spin properties, thus providing a more physically realistic description of electronic behavior. The model incorporates quantum kinetic energy, Coulomb integration, and Pauli repulsion of electrons by introducing specialized potential functions applicable to different types of wave packets, thereby superseding the traditional bond-order concept. The EeFF has been optimized using training sets of CH compounds with more than 10000 conformations covering a wide range of hydrocarbon systems and reactions. Compared with ReaxFF, EeFF reduces energy and force errors. A new molecular dynamics simulation method is used to realize the dynamic evolution of electron spin properties and allow the degrees of freedom of nuclei and electrons to change over time while approaching adiabatic conditions. Subsequent simulations of iso-octane thermal decomposition effectively reproduce the main product distributions as well as the key reaction pathways, and the motion trajectories of electrons provide unique insights into the reaction mechanisms at the electron level.