  Two models for the calculation of ionization cross sections by electron
impact on atoms, the Binary-Encouter-Bethe and the Deutsch-Maerk models, have
been implemented; they are intended to extend and improve Geant4 simulation
capabilities in the energy range below 1 keV. The physics features of the
implementation of the models are described, and their differences with respect
to the original formulations are discussed. Results of the verification with
respect to the original theoretical sources and of extensive validation with
respect to experimental data are reported. The validation process also concerns
the ionization cross sections included in the Evaluated Electron Data Library
used by Geant4 for low energy electron transport. Among the three cross section
options, the Deutsch-Maerk model is identified as the most accurate at
reproducing experimental data over the energy range subject to test.
