The ALICE Geant4 Simulation

International audienceALICE adopted a strategy to develop a virtual interface to the detector simulation codes, the Virtual Monte Carlo [1], with which the transport of particles can be performed with three different detector simulation codes: GEANT 3.21 [2], Geant4 [3], and FLUKA [4]. The Root geometrical modeller, TGeo [5], was adopted by ALICE as the unique geometry description in the simulation and reconstruction framework. This implied the integration of the TGeo geometrical modeller with all the transport codes used. GEANT3 was the most frequently used detector transport codes in past years, however, the recent LHC data production has created a greater interest in other transport codes. In this paper we will present our experience with Geant4. We will give an overview and the present status of the tools used in the Geant4 simulation: the implementation of the Virtual Monte Carlo interface, Geant4 VMC [6], and the implementation of Geant4 geometry navigation using directly the TGeo geometry, G4Root [7]. We will also present the deployment of these tools on the Grid, the results obtained, as well as their comparison with GEANT3 and with real data