*Review* **ATOM Program System and Computational Experiment**

**Larissa V. Chernysheva 1,\* and Vadim K. Ivanov 2,\***

	- **\*** Correspondence: larissa.chernysheva@mail.ioffe.ru (L.V.C.); ivanov@physics.spbstu.ru (V.K.I.)

**Abstract:** The article is devoted to a brief description of the ATOM computer program system, designed to study the structure, transition probabilities and cross sections of various processes in multielectron atoms. The theoretical study was based on the concept of a computational experiment, the main provisions of which are discussed in the article. The main approximate methods used in the system of programs for taking many-electron correlations into account and determining their role in photoionization processes, elastic and inelastic electron scattering, the decay of vacancies, and many others are presented. The most significant results obtained with this software are listed.

**Keywords:** atom; RPAE; photoionization; electron-impact ionization; ionization of the inner shells; decay of vacancies; computer
