Many-body molecular dynamics force fields for chemistry at extreme conditions*

Molecular materials at high pressures and temperatures, such as those produced by a shock wave or detonation, often undergo rapid chemical reactions. There has been significant progress in understanding chemical processes at extreme conditions through simulations based on Kohn-Sham density function theory (DFT). DFT simulations, however, are typically limited in size to less than 1000 atoms and in time to less than 100 ps. We have developed a reactive molecular dynamics force field, called ChIMES, that retains much of the accuracy of density functional theory, while allowing application to much larger systems and longer time scales. The ChIMES approach has been recently extended to treat 4 body interactions and charge fluctuations. We find that these additions significantly enhance the accuracy of the force field, but require more sophisticated calibration methods to maintain accuracy and stability for a wide range of conditions. Effective calibration methods will be presented based on techniques adapted from the machine learning community. Applications to liquid carbon, H₂O, and N₃H will be discussed.