Fast simulation for ATLAS: Atlfast-II and ISF

We present the ATLAS simulation packages ATLFAST-II and ISF. Atlfast-II is a sophisticated fast parametrized simulation in the Calorimeter system in combination with full Geant4 simulation precision in the Inner Detector and Muon Systems. This combination offers a relative increase in speed of around a factor of ten compared to the standard ATLAS detector simulation and is being used to supplement the ATLAS MC needs for 2011. The design of the parametrized simulation components allows for a flexible tuning of the simulated detector response to direct measurements with the ATLAS detector in order to increase the overall simulation quality. Hence the tuned fast simulation approach promises to be usable for the production of large MC samples needed for new physics searches as well as precision measurements. The newly developed highly configurable ATLAS Integrated Simulation Framework (ISF) extends Atlfast-II by fast inner detector and muon system simulation. Furthermore, depending on the required accuracy, differently flavored full and fast simulation approaches can be mixed within a single event to allow an optimal balance between precision and execution time. The ISF is built into the ATLAS event data processing framework Athena and is designed to allow for future extension and the application of parallel computing techniques.