G3 to G4
Multiple scattering

Step constraint

• Geant3
  Bethe Moliere formalism.
  At low energy a small angle deviation impose a step limitation.
  

• Geant4
  New model from L. Urban
  No constraint on the step: convert a "true path length" into a "geometrical path length"
  

Final state

• Geant3
  Sample the angle from Bethe Moliere distribution.
  Compute an approximate path length correction.
  

• Geant4
  Sample the angle from the L. Urban model, including a semi-empirical formula for the nuclear size correction.
  Convert "geometrical" to "true" path length. (equivalent to a path length correction)
  Perform a lateral displacment.