Calculating the IMFP with either the TTP-2M formula or the S1 formula requires different knowledge of some parameters. Applying the TTP-2M formula one needs to know M , ρ ρ and N ν ν for conducting materials (and also E g for non-conductors). Employing S1 formula, knowledge of the atomic number Z (average atomic number for compounds), M and ρ ρ is required for conductors. If non-conducting materials are considered, one also needs to know either E g or H .