Optimizing the ATLAS Geant4 detector simulation software

The ATLAS experiment at the LHC relies critically on simulated event samples produced by the full Geant4 detector simulation software FullSim. FullSim was the major CPU consumer during the last data-taking year in 2018 and it is expected to be still significant in the HL-LHC era. In September 2020 ATLAS formed the Geant4 Optimization Task Force to optimize the computational performance of FullSim for the Run 3 Monte Carlo campaign. This report summarizes the already implemented and upcoming improvements. These include improved features from the core Geant4 software, optimal options in the simulation configuration, simplifications in geometry and magnetic field description and technical improvements in the way ATLAS simulation code interfaces with Geant4 Overall, more than 50% higher throughput is achieved, compared to the baseline simulation configuration used during Run 2.