From da083d7f3e7d95469a697d5e73105dae420e13b5 Mon Sep 17 00:00:00 2001
From: Sandro Wenzel <sandro.wenzel@cern.ch>
Date: Tue, 17 Feb 2026 19:47:42 +0100
Subject: [PATCH] o2-sim: Possibility to switch between TGeo and Geant4
 navigation

---
 Common/SimConfig/include/SimConfig/G4Params.h |  9 +++++++++
 Common/SimConfig/src/SimConfigLinkDef.h       |  1 +
 Detectors/gconfig/g4Config.C                  | 11 ++++++++++-
 Steer/src/O2MCApplication.cxx                 |  6 ++++++
 4 files changed, 26 insertions(+), 1 deletion(-)

diff --git a/Common/SimConfig/include/SimConfig/G4Params.h b/Common/SimConfig/include/SimConfig/G4Params.h
index fd36ae046d520..aa8aa05263c0a 100644
--- a/Common/SimConfig/include/SimConfig/G4Params.h
+++ b/Common/SimConfig/include/SimConfig/G4Params.h
@@ -33,6 +33,13 @@ enum class EG4Physics {
   kUSER = 8                           /* allows to give own string combination */
 };
 
+// enumerating possible geometry navigation modes
+// (understanding that geometry description is always done with TGeo)
+enum class EG4Nav {
+  kTGeo = 0, /* navigate with TGeo */
+  kG4 = 1    /* navigate with G4 native geometry */
+};
+
 // parameters to influence the G4 engine
 struct G4Params : public o2::conf::ConfigurableParamHelper<G4Params> {
   EG4Physics physicsmode = EG4Physics::kFTFP_BERT_EMV_optical; // default physics mode with which to configure G4
@@ -40,6 +47,8 @@ struct G4Params : public o2::conf::ConfigurableParamHelper<G4Params> {
   std::string configMacroFile = ""; // a user provided g4Config.in file (otherwise standard one fill be taken)
   std::string userPhysicsList = ""; // possibility to directly give physics list as string
 
+  EG4Nav navmode = EG4Nav::kTGeo; // geometry navigation mode (default TGeo)
+
   std::string const& getPhysicsConfigString() const;
 
   O2ParamDef(G4Params, "G4");
diff --git a/Common/SimConfig/src/SimConfigLinkDef.h b/Common/SimConfig/src/SimConfigLinkDef.h
index 9c27536be5eb8..a1315e24ffedd 100644
--- a/Common/SimConfig/src/SimConfigLinkDef.h
+++ b/Common/SimConfig/src/SimConfigLinkDef.h
@@ -29,6 +29,7 @@
 #pragma link C++ class o2::conf::ConfigurableParamHelper < o2::conf::DigiParams> + ;
 
 #pragma link C++ enum o2::conf::EG4Physics;
+#pragma link C++ enum o2::conf::EG4Nav;
 #pragma link C++ enum o2::conf::SimFieldMode;
 #pragma link C++ struct o2::conf::G4Params + ;
 #pragma link C++ class o2::conf::ConfigurableParamHelper < o2::conf::G4Params> + ;
diff --git a/Detectors/gconfig/g4Config.C b/Detectors/gconfig/g4Config.C
index 8f74c0105dbf5..c2b1fbd433e4b 100644
--- a/Detectors/gconfig/g4Config.C
+++ b/Detectors/gconfig/g4Config.C
@@ -100,7 +100,16 @@ void Config()
   auto& g4Params = ::o2::conf::G4Params::Instance();
   auto& physicsSetup = g4Params.getPhysicsConfigString();
   std::cout << "PhysicsSetup wanted " << physicsSetup << "\n";
-  auto runConfiguration = new TG4RunConfiguration("geomRoot", physicsSetup, "stepLimiter+specialCuts",
+  std::string geomNavStr;
+  if (g4Params.navmode == o2::conf::EG4Nav::kTGeo) {
+    geomNavStr = "geomRoot";
+  } else if (g4Params.navmode == o2::conf::EG4Nav::kG4) {
+    geomNavStr = "geomVMC+RootToGeant4";
+  } else {
+    LOG(fatal) << "Unsupported geometry navigation mode";
+  }
+
+  auto runConfiguration = new TG4RunConfiguration(geomNavStr, physicsSetup, "stepLimiter+specialCuts",
                                                   specialStacking, mtMode);
   /// avoid the use of G4BACKTRACE (it seems to inferfere with process logic in o2-sim)
   setenv("G4BACKTRACE", "none", 1);
diff --git a/Steer/src/O2MCApplication.cxx b/Steer/src/O2MCApplication.cxx
index f832ab70ab121..1e3f925042d01 100644
--- a/Steer/src/O2MCApplication.cxx
+++ b/Steer/src/O2MCApplication.cxx
@@ -42,6 +42,7 @@
 #include <TGeoTessellated.h>
 #include <DetectorsBase/O2Tessellated.h>
 #include <unordered_set>
+#include "SimConfig/G4Params.h"
 
 namespace o2
 {
@@ -223,6 +224,11 @@ bool O2MCApplicationBase::MisalignGeometry()
 
 void O2MCApplicationBase::fixTGeoRuntimeShapes()
 {
+  auto& g4Params = o2::conf::G4Params::Instance();
+  if (g4Params.navmode != o2::conf::EG4Nav::kTGeo) {
+    return;
+  }
+
   // Replace TGeo shapes by other ones for performance or other reasons.
   // Should only affect runtime of simulation.