# Stepper accuracy: parameters involved¶

`ExN02MagneticField.cc`

- 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`

.

- Inside the class constructor we can find the following accuracy parameters:

- Default values
- These can be found at
`geometry/magneticfield/src/G4FieldManager.cc`

- 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)

- This file provides the implementation of the
`geometry/magneticfield/src/G4ChordFinder.cc`

- Implementation of class
`G4ChordFinder`

(where the miss distance is set) - Again, the constructors specify the default value for this parameter:
`fDefaultDeltaChord`

: 0.25 mm

- Implementation of class
`stepMax`

is set to 100 mm, by default, during the detector construction (see source:N02to/src/ExN02DetectorConstruction.cc@0ab505d2#L206).`G4UserLimits`

class uses`DBL_MAX`

as default value, but this is never used in our code.

- These can be found at

- 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)

- These slides briefly explain the purpose of the
- References
- Slides showing default units: http://www.tifr.res.in/~gobinda/inosim/geant_ino2013.pdf
- Short explanation of
`epsilon max`

parameter: http://geant4.lngs.infn.it/corso_infn/slides/field.pdf - John's talk on propagation in the field in Geant4: http://geant4.in2p3.fr/2007/prog/JohnApostolakis/EMField.pdf