The default Geant4 VMC behaviour, defined by the Geant4 user mandatory classes and user action classes implemented in Geant4 VMC, can be customized by a user by providing his own class derived from TG4RunConfiguration.
Such customization is recommended for including a user own physics list. User has also the possibility to override detector construction and/or primary generation action classes and use an existing Geant4 geometry/primary generator definition with VMC. In other cases, though the customisation is possible and allowed by the design, it has not been tested and so it is not recommended especially for the novice users.
The example of including user own physics list is provided in the VMC example E03 in Ex03RunConfiguration2 class.
In case, a user has registered his own physics list, he has a possibility to combine his own physics list with the TG4SpecialPhysicsList using TG4ComposedPhysics list as it is demonstrated in the VMC example E03, see Ex03RunConfiguration2, and he can then activate any special process as described above.( Another possibility, which requires more expertise, would be to register the special process physics constructor (eg. class TG4SpecialCutsPhysics) in a user own modular physics list.)
Including of user geometry construction is also demonstrated in the example E03 in Ex03RunConfiguration1 class.
Since version 2.5, users have a possibility to define Geant4 regions by overriding the TG4VUserRegionConstruction class. Definition and including of such a user region construction class in the VMC application is demonstrated in the example E03 in Ex03RunConfiguration3 class.