to the Multiple Scattering:
Multiple Scattering electromagnetic process
In the electromagnetic standard package major changes have been introduced concerning the Multiple Scattering process. To improve the behaviour of low energy particles (electrons in particular, but affecting also hadrons), the Multiple Scattering now limits the step size for the particles. This restriction is undertaken using several criteria, and is applied systematically, in all volumes and materials. In addition a model of the correlation between lateral displacement and final direction has been implemented (see the Physics Reference Manual for further information).
As a result, most physical observables become more stable when varying production cuts (i.e. less "cut dependent"). There is a corresponding cost, a CPU-time penalty, when utilising the same value of the production thresholds. This penalty can be significant, depending on the user's setup and the cut values. For many use cases the increased stability will allow the choice of higher production thresholds, recovering computing performance while maintaining physics performance.
To enable the user to investigate its benefits, a mechanism is provided to deactivate this step limitation. The new method MscStepLimitation(bool) of G4MultipleScattering disables these new step limitations. In addition, in order to help the transition, the old version is available for this release 'frozen' in the class G4MultipleScattering71. Several examples (in extended/electromagnetic) provide sample physics lists which use this older version.