First-Principles Simulation of Ionizing Radiation Damage in Molecular Systems over Short Time Scales

In this work, we present an ab initio molecular simulation method designed to investigate the ultrashort time scales of radiation chemistry induced by ionizing radiation, in particular fast ions and electric pulses. The approach accounts for energy deposition at the subfemtosecond scale, electron emission, and energy dissipation into nuclear vibrational modes, which may ultimately lead to the formation of the earliest radiation-induced damages. The method, implemented in deMon2k, is based on the coupling between Real-Time Time-Dependent Density Functional Theory (RT-TDDFT) within the Auxiliary DFTformalism and Ehrenfest mean-field dynamics. Complex absorbing potentials are introduced to cope with emitted electrons. A hybrid QM/MM scheme further enables the simulation of radiation damage formation in complex and heterogeneous systems such as proteins or DNA. The method has been tested on a set of benchmark molecules and achieves controlled numerical accuracy down to the μeV scale for the total energy conservation. From a computational efficiency perspective, the implementation relies on the Auxiliary DFT formalism and fully benefits from modern CPU- and GPU-based computer architectures. As an illustrative application, we present simulations of a solvated tributyl phosphate molecule subjected to alpha radiation, a problem of direct relevance to the nuclear industry. The study analyzes the energy deposition mechanisms, the dissipation pathways, and the characterization of the earliest radiation-induced damages. In summary, the ionizing-radiation chemistry module of deMon2k presented here opens the way toward a deeper understanding of the ultrafast formation of radiation-induced chemical damages in fields as diverse as biology, the nuclear industry, aerospace science, and astrochemistry.