G3toG4 is the Geant4 facility to convert GEANT 3.21 geometries into Geant4. This is done in two stages: The user supplies a GEANT 3.21 RZ-file (.rz) containing the initialization data structures. An executable rztog4 reads this file and produces an ASCII call list file containing instructions on how to build the geometry. The source code of rztog4 is FORTRAN. A call list interpreter (G4BuildGeom.cc) reads these instructions and builds the geometry in the user's client code for Geant4. Two examples of how to use the call list interpreter are supplied in the directory examples/extended/g3tog4: cltog4 is a simple example which simply invokes the call list interpreter method G4BuildGeom from the G3toG4DetectorConstruction class, builds the geometry and exits. clGeometry, is more complete and is patterned as for the novice Geant4 examples. It also invokes the call list interpreter, but in addition, allows the geometry to be visualized and particles to be tracked. To compile and build the G3toG4 libraries, you need to have set in your environment the variable G4LIB_BUILD_G3TOG4 at the time of installation. The G3toG4 libraries are not built by default. Then, simply type gmake from the top-level source/g3tog4 directory. To build the converter executable rztog4, simply type gmake bin To make everything, simply type: gmake global To remove all G3toG4 libraries, executables and .d files, simply type gmake clean The package has been tested with the geometries from experiments like: BaBar, CMS, Atlas, Alice, Zeus, L3, and Opal. Here is a comprehensive list of features supported and not supported or implemented in the current version of the package: Supported shapes: all GEANT 3.21 shapes except for GTRA, CTUB. PGON, PCON are built using the specific solids G4Polycone and G4Polyhedra. GEANT 3.21 MANY feature is only partially supported. MANY positions are resolved in the G3toG4MANY() function, which has to be processed before G3toG4BuildTree() (it is not called by default). In order to resolve MANY, the user code has to provide additional info using G4gsbool(G4String volName, G4String manyVolName) function for all the overlapping volumes. Daughters of overlapping volumes are then resolved automatically and should not be specified via Gsbool. Limitation: a volume with a MANY position can have only this one position; if more than one position is needed a new volume has to be defined (gsvolu()) for each position. GSDV* routines for dividing volumes are implemented, using G4PVReplicas, for shapes: BOX, TUBE, TUBS, PARA - all axes; CONE, CONS - axes 2, 3; TRD1, TRD2, TRAP - axis 3; PGON, PCON - axis 2; PARA -axis 1; axis 2,3 for a special case GSPOSP is implemented via individual logical volumes for each instantiation. GSROTM is implemented. Reflections of hierachies based on plain CSG solids are implemented through the G3Division class. Hits are not implemented. Conversion of GEANT 3.21 magnetic field is currently not supported. However, the usage of magnetic field has to be turned on.