At the end of a free flight, a scattering mode and angle must be randomly chosen. In order to determine the scattering mechanism, one has to consider all the scattering rates λ λ 1 , λ λ 2 , . . . , λ λ n of the mechanisms relevant to the simulation as well as the total scattering rate at the time of scattering λ λ t o t ( t s c ) = ∑ ∑ i λ λ i . Selecting a scattering mechanism then simply results in generating a uniformly distributed random number 0 < r < 1 and referring to the following rules