Here, φ φ ( r ) → → 1 when r → → 0 . Z 1 and Z 2 are the charges of the interacting nuclei, and a is the so-called screening parameter. A widely used popular screening function is the "Universal ZBL" one. and more accurate ones can be obtained from all-electron quantum chemistry calculations In binary collision approximation simulations this kind of potential can be used to describe the nuclear stopping power.