The process of electron impact ionization of atom is one of the key processes determining the main properties of low temperature plasma. The calculation of the cross section of this process can be done in the framework of the comprehensive quantum mechanical approach. However this approach meets difficulties inherent to the many body problem. These difficulties can be overcome using the simple but reliable similarity function method. The similarity function method provides the accuracy in description of the electron impact ionization cross sections within the characteristic experimental spread. This method based on the assumption that the ionization cross section of given atomic subshell depends on the dimensionless ratio between the energy of incident electron and ionization potential of the subshell. Similarity function (SF) satisfies near-threshold and high-energy boundary conditions. The first one is defined by general quantum-mechanical law and the second is determined by Born approximation which is valid in high-energy limit. There are several types of similarity function: Gryzhinsky SF, Eletskii-Smirnov SF, binary-encounter-Born (BEB) SF, Born-Compton SF. Each of SF has the range of the best applicability. So Eletskii-Smirnov SF is most preferable for target subshell with high ionization potential; Born-Compton approach gives better results for atoms from the middle part of the Periodic Table possessing the moderate value of ionization potential; BEB approximation is applicable in the case of high orbital quantum number of released electrons. Atomic parameters needed to realize the calculations in the frame of models mentioned above are well known from the textbooks. They are the ionization potential and number of equivalent electrons in ionized subshell.

Similarity function method in the same form can also be used for the calculation of the molecular ionization cross section by electron impact in the frame of molecular orbital approximation when the ionization potential and the number of equivalent electrons in each molecular orbital are known.

Figures below demonstrate the calculation results obtained in the frame-work of above-mentioned methods together with available experimental data.