Large-scale Electron Dynamics Simulations of Periodic Systems Using Real-Time Time-Dependent Density Functional Theory (RT-TDDFT)

We present the implementation and benchmarking of the real-time time-dependent DFT (RT-TDDFT) module within the first-principles Real-Space Multigrid (RMG) package [JCTC 21, 1322 (2025)] for simulating the electronic response of periodic systems to external perturbations. We benchmark our computed optical absorption spectra for (a) a one-dimensional hydrogen dimer, (b) a two-dimensional MoS₂ monolayer, and (c) a three-dimensional bulk silicon crystal, against existing TDDFT implementations in established DFT packages, and achieve excellent agreement. We further demonstrate the scalability and efficiency of RMG on exascale supercomputing architectures, such as Frontier at ORNL, by simulating large systems with a few thousand atoms and a few tens of thousands of electrons. This superior scalability allows us to systematically investigate the effects of defect densities on the absorption spectra in monolayer 2D materials (e.g., S vacancies in MoS₂; V_B^- spin quibit vacancies in hBN), reaching realistic defect concentrations in experiments where large supercells are required in simulations. Generally, our massively parallelized TDDFT implementation provides a powerful toolset for studies of non-equilibrium responses in photoactive materials, nanoscale devices, and other systems where real-time electron dynamics is essential.