The VMC support two ways of geometry definition:

• via Root geometry package (TGeo)
• via TVirtualMC interface (historically the first way)

The first (newer) way is recommended for new users, the way via VMC is kept for a backward compatibility.

Since the version 2.0, user can choose between Geant4 native navigation and G4Root navigation, if geometry is define via TGeo. The choice of the navigation is done via the option specified with creating TG4RunConfiguration object (see examples for more details):

• 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

Below we shortly comment the implementation of these options:

• geomVMCtoGeant4
  The interfaces to functions for geometry definitions provided by VMC were strongly inspired by Geant3; the implementation of these functions in Geant4 VMC is therefore made with use of the G3toG4 tool provided by Geant4.
• geomVMCtoRoot
  The implementation of this option uses TGeoMCGeometry class provided in the root/vmc package. The same class is used also by TGeant3TGeo and TFluka for supporting user geometry defined via VMC.
• geomRoot
  If geometry is defined via Root and G4Root navigation is selected, Geant4 VMC only converts the parameters defined in TGeoMedium objects to Geant4 objects.
• geomRootToGeant4
  The geometry defined via TGeo is converted in Geant4 geometry using the external Virtual Geometry Model (VGM), which has replaced the old one-way converters from Geant4 VMC (G4toXML, RootToG4), removed from Geant4 VMC with the version 1.7. In the VGM, these convertors has been generalized and improved.
• geomGeant4
  User Geant4 detector construction class can be passed to Geant4 VMC via user defined run configuration class (see this page ). If Geant4 VMC is built with VGM, geometry can be exported in Root using the built-in command:
  

  /vgm/generateRoot 

  and reused in Geant3 or Fluka (when available) VMC simulation.

Useful commands

/mcDet/printMaterials
/mcDet/printMaterialsProperties
/mcDet/printMedia
/mcDet/printVolumes
   prints materials, material properties, tracking media, volumes

In Geant4 VMC, there is by default set a step limit 10 cm for all materials with density lower than 0.001 g/cm3. In the development version (SVN trunk), this default setting can be overridden via the following commands:

/mcDet/setLimitDensity 1.e-6 g/cm3
/mcDet/setMaxStepInLowDensityMaterials 1 mm

How to apply Geant4 commands in a Root user session is explained at the section on User Interfaces.

Geometry in XML

The VGM provides also the XML exporters which enable to generate XML files with geometry description in the AGDD or GDML formats. If Geant4 VMC is compiled with USE_VGM option, geometry can be exported to XML using the built-in commands:

/vgm/generateAGDD [volumeName]
/vgm/generateGDML [volumeName]

The geometry can be then browsed and visualized using GraXML tool .

"MANY" positions

As Geant4 does not support overlapping geometries, the positions with MANY option are not allowed with Geant4 navigation.

In case of geomVMCtoGeant4 option, user has a possibility to identify all ONLY volumes that overlap with the MANY ones using TVirtualMC::Gsbool() function for each overlap. This info is then used to perform automatically Boolean operations on the concerned solids. The volume with a "MANY" position can have only this position if Gsbool is used.

In case of geomRootToGeant4 option, the MANY option is ignored and if present in geometry, Gean4 geometry will be incorrect.

There is no limitation on use of the MANY option with G4Root navigation (options geomVMCtoRoot, geomRoot).