Reproducibility and nonlinear effects in TDDFT simulations of stopping power in warm dense matter

First-principles models like time-dependent density functional theory (TDDFT) enjoy special reverence in the warm dense regime, where scarcity of experimental data makes them particularly valuable benchmarks for constraining and characterizing limitations of more efficient methods. However, TDDFT calculations themselves involve many choices, including convergence parameters, approximations for both electron-electron and electron-ion interactions, details of the time-dependent perturbation applied, and analysis techniques to extract meaningful observables from the electronic response. Here, we cross-benchmark different TDDFT codes and methodologies for computing electronic stopping power of charged particles in warm dense matter, an important transport coefficient governing self-heating in inertial confinement fusion. After assessing reproducibility across codes and scrutinizing best practices for the case of alpha particle stopping in warm dense hydrogen, we consider nonlinear effects in the form of deviations from Z² scaling between proton and alpha stopping in warm dense carbon and aluminum. This work boosts confidence in TDDFT stopping power results and reveals opportunities to improve effective charge models for more efficient linear-response treatments.