In the so-called Monte Carlo BCA approach the distance to and impact parameter of the next colliding atom is chosen randomly from a probability distribution which depends only on the atomic density of the material. This approach essentially simulates ion passage in a fully amorphous material. (Note that some sources call this variety of BCA just Monte Carlo, which is misleading since the name can then be confused with other completely different Monte Carlo simulation varieties). SRIM and SDTrimSP are Monte-Carlo BCA codes.