Entering the simulation domain an ion starts interacting with the target material, which can be simulated by successively applying nuclear and electronic stopping processes (Sec. 3.3). Therefore it is necessary to locate atomic cores in the target material which collide with the implanted particle. By knowing the collision partners the initial conditions for a nuclear stopping process (mass and charge of the target and the energy and the impact parameter of the projectile) and the free flight path length (distance between two scattering processes) for an electronic stopping process are determined. As the simulator handles crystalline and amorphous materials two different selection strategies are used.