The ATLAS simulation in preparation for the LHC startup

The commissioning phase for the ATLAS experiment, now in preparation of the switching on of the new Large Hadron Collider (LHC) machine, is a challenge for every development since now performed. The simulation program is in this line, it is operational and running with full functionality within the ATLAS common framework Athena. The latest developments are towards an enhanced versatility, to cope with the increasing needs of developers and users, and easiness in use by a big community such as ATLAS reaching now more than 2000 potential users. Emphasis in this talk is towards the new functionality recently added, validation and production strategy as well as improved robustness and maintainability.