I want to simulate the scattering in a CT scan. I simulate 100 million photons （“gamma”） passing through 100mm thick water and count the number of scattered photons received on the detector. At first, I used Geant4, but this simulation is too slow. Instead, I used GGEMS. In the same simulation environment, the scattering simulated by Geant4 is different from that of GGEMS. Is the scattering physical process of GGEMS different from that of GEANT4 in principle?