ATLAS detector simulation: status and outlook

The simulation program for the ATLAS experiment at CERN is currently in a full operational mode and integrated into the common analysis framework of ATLAS, Athena. The robustness of the application was proved during Data Challenge 2 (DC2). The simulation software, based on GEANT4, has been interfaced within Athena and to GEANT4 itself using the LCG dictionaries and Python scripting. The Python interface has added the flexibility, modularity and interactivity that the simulation tool needs to manage, in a common way, the different simulation setups: full ATLAS detector, test beams and cosmic ray studies. The comparison with real data has been possible in the context of the ATLAS Combined Test Beam (2004) and ongoing cosmic ray studies. This paper presents a report on the status of the simulation, together with an overview of the applications and the results of the validation efforts.