We discuss different ways of calculating ion fractions for LEIS-type experiments: (i) parametrizations, and (ii) quasi-ab-initio models. Among the different parametrizations of the neutralization rates the Hagstrum, Godfrey-Woodruff and the unified model are compared. For surfaces containing different types of atoms, e.g. NaCl, it is not possible to determine the neutralization constants uniquely. This is related to the use of relative ion fractions. For the absolute ion fractions there is a convincing agreement between DISC-experiments for He+ → Si scattering and a theoretical calculation on the basis of the quasi-ab-initio Muda-Newns model.