// 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("/optics_engine/selectOpProcess Cerenkov");
  geant4->ProcessGeantCommand("/optics_engine/setTrackSecondariesFirst false");

/// create the Specific stack
   PndStack *stack = new PndStack(1000); 
   stack->StoreSecondaries(kTRUE);
   stack->SetMinPoints(2);
   geant4->SetStack(stack);

   if(FairRunSim::Instance()->IsExtDecayer()){
      TVirtualMCDecayer* decayer = TPythia6Decayer::Instance();
      geant4->SetExternalDecayer(decayer);
   }
  
/// Customise Geant4 setting
/// (verbose level, global range cut, ..)

   TString configm(gSystem->Getenv("VMCWORKDIR"));
   configm1 = configm + "/gconfig/g4config.in";
   cout << " -I g4Config() using g4conf  macro: " << configm1 << endl;

     //set geant4 specific stuff
   geant4->SetMaxNStep(10000);  // default is 30000

  Text_t buffer[50];
  sprintf(buffer,"/random/setSeeds %i  %i ",gRandom->GetSeed(), gRandom->GetSeed());
  geant4->ProcessGeantCommand(buffer);
 
  geant4->ProcessGeantMacro(configm1.Data());


}