Stepper accuracy: parameters involved

    • Inside the class constructor we can find the following accuracy parameters:
      • delta one step: 1.0e-6 nm
      • delta intersection: 1.0e-9 nm
      • minimum step size: 1.0e-2 mm
      • epsilon min and epsilon max appear commented out, which implies that their default values might be used (see below).
      • the same goes for miss distance.
  • Default values
    • These can be found at geometry/magneticfield/src/
      • This file provides the implementation of the G4FieldManager class. Setters and getters for deltas and epsilons are defined.
      • Looking at the constructors, we can observe the default values in use:
        • fDefault_Delta_One_Step_Value: 0.01 mm
        • fDefault_Delta_Intersection_Val: 0.001 mm
        • fEpsilonMinDefault: 5.0e-5 mm (units are missing, but these slides show G4's default units)
        • fEpsilonMaxDefault: 0.001 mm (units missing for this one as well)
    • geometry/magneticfield/src/
      • Implementation of class G4ChordFinder (where the miss distance is set)
      • Again, the constructors specify the default value for this parameter:
        • fDefaultDeltaChord: 0.25 mm
    • stepMax is set to 100 mm, by default, during the detector construction (see source:N02to/src/
      • G4UserLimits class uses DBL_MAX as default value, but this is never used in our code.
  • Further details
    • These slides briefly explain the purpose of the epsilon max parameter:
      The MaximumEpsilonStep parameter is the relative accuracy for the endpoint of 'ordinary' integration steps, those which do not intersect a volume boundary. Limits estimated error |Delta x| of endpoint of each physics step (of length len): |Delta x| < Epsilon * len
    • The other set of slides mentioned above contain the following default units, which might eventually come in handy:
      • Length: mm
      • Time: ns
      • Energy: MeV
      • Angle: Radian
      • Temperature: Kelvin
    • All units are defined at source/externals/clhep/include/CLHEP/Units/SystemOfUnits.h (which also specifies the basic (default) units)
  • References