ATLAS electromagnetic barrel photon testbeam response compared to GEANT4 simulation

A GEANT4 simulation of photon testbeam setup has been built in order to study the data taken in August 2000 with ECAL barrel module 0. This simulation takes into account the material found in the beam in 2001. This note describes the simulation and presents a comparison between data and GEANT4 for different variables relevant to photon identification. This work validates the GEANT4 module 0 simulation for most of these parameters, in particular for pion/photon rejection variables, and could be considered as a first step in GEANT4 validation of Ecal barrel photon response.