A Geant4 application can be run either in `purely hard-coded` batch mode batch mode, but reading a macro of commands interactive mode, driven by command lines interactive mode via a Graphical User Interface The last mode will be covered in . The first three modes are explained here. Below is an example of the main program for an application which will run in batch mode. int main() { // Construct the default run manager G4RunManager* runManager = new G4RunManager; // set mandatory initialization classes runManager->SetUserInitialization(new ExN01DetectorConstruction); runManager->SetUserInitialization(new ExN01PhysicsList); // set mandatory user action class runManager->SetUserAction(new ExN01PrimaryGeneratorAction); // Initialize G4 kernel runManager->Initialize(); // start a run int numberOfEvent = 1000; runManager->BeamOn(numberOfEvent); // job termination delete runManager; return 0; } Even the number of events in the run is `frozen`. To change this number you must at least recompile main(). Below is an example of the main program for an application which will run in batch mode, but reading a file of commands. int main(int argc,char** argv) { // Construct the default run manager G4RunManager * runManager = new G4RunManager; // set mandatory initialization classes runManager->SetUserInitialization(new MyDetectorConstruction); runManager->SetUserInitialization(new MyPhysicsList); // set mandatory user action class runManager->SetUserAction(new MyPrimaryGeneratorAction); // Initialize G4 kernel runManager->Initialize(); //read a macro file of commands G4UImanager * UI = G4UImanager::getUIpointer(); G4String command = "/control/execute "; G4String fileName = argv[1]; UI->applyCommand(command+fileName); delete runManager; return 0; } This example will be executed with the command: > myProgram run1.mac where myProgram is the name of your executable and run1.mac is a macro of commands located in the current directory, which could look like: # # Macro file for "myProgram.cc" # # set verbose level for this run # /run/verbose 2 /event/verbose 0 /tracking/verbose 1 # # Set the initial kinematic and run 100 events # electron 1 GeV to the direction (1.,0.,0.) # /gun/particle e- /gun/energy 1 GeV /run/beamOn 100 Indeed, you can re-execute your program with different run conditions without recompiling anything. Digression: many G4 category of classes have a verbose flag which controls the level of 'verbosity'. Usually verbose=0 means silent. For instance /run/verbose is for the RunManager /event/verbose is for the EventManager /tracking/verbose is for the TrackingManager ...etc... Below is an example of the main program for an application which will run interactively, waiting for command lines entered from the keyboard. int main(int argc,char** argv) { // Construct the default run manager G4RunManager * runManager = new G4RunManager; // set mandatory initialization classes runManager->SetUserInitialization(new MyDetectorConstruction); runManager->SetUserInitialization(new MyPhysicsList); // visualization manager G4VisManager* visManager = new G4VisExecutive; visManager->Initialize(); // set user action classes runManager->SetUserAction(new MyPrimaryGeneratorAction); runManager->SetUserAction(new MyRunAction); runManager->SetUserAction(new MyEventAction); runManager->SetUserAction(new MySteppingAction); // Initialize G4 kernel runManager->Initialize(); // Define UI terminal for interactive mode G4UIsession * session = new G4UIterminal; session->SessionStart(); delete session; // job termination delete visManager; delete runManager; return 0; } This example will be executed with the command: > myProgram where myProgram is the name of your executable. The G4 kernel will prompt: Idle> and you can start your session. An example session could be: Create an empty scene ("world" is default): Idle> /vis/scene/create Add a volume to the scene: Idle> /vis/scene/add/volume Create a scene handler for a specific graphics system. Change the next line to choose another graphic system: Idle> /vis/sceneHandler/create OGLIX Create a viewer: Idle> /vis/viewer/create Draw the scene, etc.: Idle> /vis/scene/notifyHandlers Idle> /run/verbose 0 Idle> /event/verbose 0 Idle> /tracking/verbose 1 Idle> /gun/particle mu+ Idle> /gun/energy 10 GeV Idle> /run/beamOn 1 Idle> /gun/particle proton Idle> /gun/energy 100 MeV Idle> /run/beamOn 3 Idle> exit For the meaning of the machine state Idle, see . This mode is useful for running a few events in debug mode and visualizing them. Notice that the VisManager is created in the main(), and the visualization system is choosen via the command: /vis/sceneHandler/create OGLIX Most of the examples in the $G4INSTALL/examples/ directory have the following main(). The application can be run either in batch or interactive mode. int main(int argc,char** argv) { // Construct the default run manager G4RunManager * runManager = new G4RunManager; // set mandatory initialization classes N03DetectorConstruction* detector = new N03DetectorConstruction; runManager->SetUserInitialization(detector); runManager->SetUserInitialization(new N03PhysicsList); #ifdef G4VIS_USE // visualization manager G4VisManager* visManager = new G4VisExecutive; visManager->Initialize(); #endif // set user action classes runManager->SetUserAction(new N03PrimaryGeneratorAction(detector)); runManager->SetUserAction(new N03RunAction); runManager->SetUserAction(new N03EventAction); runManager->SetUserAction(new N03SteppingAction); // get the pointer to the User Interface manager G4UImanager* UI = G4UImanager::GetUIpointer(); if (argc==1) // Define UI terminal for interactive mode { G4UIExecutive* session = new G4UIExecutive(argc, argv); UI->ApplyCommand("/control/execute prerunN03.mac"); session->SessionStart(); delete session; } else // Batch mode { G4String command = "/control/execute "; G4String fileName = argv[1]; UI->ApplyCommand(command+fileName); } // job termination #ifdef G4VIS_USE delete visManager; #endif delete runManager; return 0; } Notice that the visualization system is under the control of the precompiler variable G4VIS_USE. Notice also that, in interactive mode, few intializations have been put in the macro prerunN03.mac which is executed before the session start. # Macro file for the initialization phase of "exampleN03.cc" # # Sets some default verbose flags # and initializes the graphics. # /control/verbose 2 /control/saveHistory /run/verbose 2 # /run/particle/dumpCutValues # # Create empty scene ("world" is default) /vis/scene/create # # Add volume to scene /vis/scene/add/volume # # Create a scene handler for a specific graphics system # Edit the next line(s) to choose another graphic system # #/vis/sceneHandler/create DAWNFILE /vis/sceneHandler/create OGLIX # # Create a viewer /vis/viewer/create # # Draw scene /vis/scene/notifyHandlers # # for drawing the tracks # if too many tracks cause core dump => storeTrajectory 0 /tracking/storeTrajectory 1 #/vis/scene/include/trajectories Also, this example demonstrates that you can read and execute a macro interactively: Idle> /control/execute mySubMacro.mac