The total non-relativistic stopping power is therefore the sum of two terms: F ( E ) = F e ( E ) + F n ( E ) . Several semi-empirical stopping power formulas have been devised. The model given by Ziegler, Biersack and Littmark (the so-called "ZBL" stopping, see next chapter), implemented in different versions of the TRIM/SRIM codes, is used most often today.