In the BCA approach, a single collision between the incoming ion and a target atom (nucleus) is treated by solving the classical scattering integral between two colliding particles for the impact parameter of the incoming ion. Solution of the integral gives the scattering angle of the ion as well as its energy loss to the sample atoms, and hence what the energy is after the collision compared to before it. The scattering integral is defined in the centre-of-mass coordinate system (two particles reduced to one single particle with one interatomic potential) and relates the angle of scatter with the interatomic potential.