// Configuration macro for Geant4 VirtualMC
void Config()
{
  ///    Create the run configuration
  /// In constructor user has to specify the geometry input
  /// and select geometry navigation via the following options:
  /// - geomVMCtoGeant4   - geometry defined via VMC, G4 native navigation
  /// - geomVMCtoRoot     - geometry defined via VMC, Root navigation
  /// - geomRoot          - geometry defined via Root, Root navigation
  /// - geomRootToGeant4  - geometry defined via Root, G4 native navigation
  /// - geomGeant4        - geometry defined via Geant4, G4 native navigation
  ///
  /// The second argument in the constructor selects physics list:
  /// - emStandard         - standard em physics (default)
  /// - emStandard+optical - standard em physics + optical physics
  /// - XYZ                - selected hadron physics list ( XYZ = LHEP, QGSP, ...)
  /// - XYZ+optical        - selected hadron physics list + optical physics
  ///
  /// The third argument activates the special processes in the TG4SpecialPhysicsList,
  /// which implement VMC features:
  /// - stepLimiter       - step limiter (default) 
  /// - specialCuts       - VMC cuts
  /// - specialControls   - VMC controls for activation/inactivation selected processes
  /// - stackPopper       - stackPopper process
  /// When more than one options are selected, they should be separated with '+'
  /// character: eg. stepLimit+specialCuts.

  TG4RunConfiguration* runConfiguration = new TG4RunConfiguration("geomRoot", "QGSP_BERT_EMV+optical","stepLimiter+specialCuts+specialControls");

  /// Create the G4 VMC 
  TGeant4* geant4 = new TGeant4("TGeant4", "The Geant4 Monte Carlo", runConfiguration);
  cout << "Geant4 has been created." << endl;
  geant4->ProcessGeantCommand("/mcVerbose/all 0"); 
  geant4->ProcessGeantCommand("/mcVerbose/eventAction 1");
  
  // geant4->ProcessGeantCommand("/optics_engine/selectOpProcess Cerenkov");
  // geant4->ProcessGeantCommand("/optics_engine/setTrackSecondariesFirst false"); 
  // geant4->ProcessGeantCommand("/optics_engine/setCerenkovMaxPhotons 5");
  // geant4->ProcessGeantCommand("/optics_engine/setCerenkovMaxBetaChange 0.1");
  
  /// create the Specific stack
  PndStack *stack = new PndStack(1000); 
  //stack->StoreSecondaries(kFALSE); //default is kTRUE
  stack->SetMinPoints(2);
  geant4->SetStack(stack);

  if(FairRunSim::Instance()->IsExtDecayer()){
    TVirtualMCDecayer* decayer = TPythia6Decayer::Instance();
    geant4->SetExternalDecayer(decayer);
  }
  
  geant4->SetMaxNStep(10000);  // default is 30000

  Text_t buffer[50];
  sprintf(buffer,"/random/setSeeds %i  %i ",gRandom->GetSeed(), gRandom->GetSeed());
  geant4->ProcessGeantCommand(buffer);
}