Please note this is a comparison between Version 1 by Larissa V Chernysheva and Version 2 by Sirius Huang.

The ATOM computer system is designed to study the structure of atoms and the physical processes occurring with their participation.

- atom
- RPAE
- photoionization
- electron-impact ionization
- computer

In the theoretical study of multielectron atomic systems, software plays an important role because it is necessary for calculating the energy and spatial structure and characteristics of various processes occurring in atoms. Calculations in the framework of relatively simple one-electron models did not lead to agreement between calculations and experimental data, despite the fact that the pair interaction between electrons is the well-known Coulomb interaction ^{[1][2][3][4][5][6][7]}[1,2,3,4,5,6,7]. In the 1960s–1970s, it became completely clear that in order to describe the structure and reaction of atoms to external influences, it is necessary to go beyond single-electron concepts and take collective multielectron effects into account ^{[3][4][5][6][7]}[3,4,5,6,7]. At the same time, in the theoretical group of the St. Petersburg Ioffe Institute, under the guidance of Prof. Miron Ya. Amusia, began to create a package of applied programs known as ATOM ^{[8][9][10]}[8,9,10], which was improved in subsequent years and aided a huge number of direct calculations that were carried out in atomic systems to describe experimental data and predict new many-particle effects. These calculations laid the foundation for the development of the concept of a computational experiment.

Theoretical studies of many-body systems, as a rule, are accompanied by the need to take many-body interactions into account. At the same time, the determination of the spatial and energy structure, as well as the response of such systems to external influences, faces significant computational difficulties, especially in cases where perturbation theory cannot be consistently applied ^{[7][8][9][10][11]}[7,8,9,10,11]. The theory of atoms and atomic particles is also facing such difficulties despite the fact that the pair interaction between electrons has been well-known to physicists for more than a century. Nevertheless, a significant development of methods for describing multielectron atoms and their interactions with external fields began in the second half of the last century when the apparatus of many-body theory was developed, great computational capabilities appeared, and experimental studies of atomic systems began to be widely carried out ^{[8][9][10]}[8,9,10]. These experiments showed that the simplest description of atoms based on single-particle models—the Hartree–Fock (HF) approximation, in particular [12]—is unable to describe the characteristics and properties of the behavior of atoms in cases of their interactions with electromagnetic fields and other particles.

One of the most successful theoretical approaches to describing the structure and processes in atoms are approximations based on many-body theory, particularly including perturbation theory [13] and random phase exchange approximation (RPAE) ^{[3][5][6]}[3,5,6]. It was the RPAE method, developed as the 1960s turned into the 1970s, that first made it possible to describe the photoionization cross sections of multielectron atoms and to show the decisive role of many-particle (collective) interactions in atomic systems. The big advantage of the RPAE is the self-consistency of this approximation, within which the main general laws of the processes of the ionization and excitation of electrons, such as the oscillator sum rule and the condition of gradient invariance, are satisfied ^{[7][8]}[7,8]. Subsequently, the application of this approximation made it possible to describe and explain numerous experimental data and predict many-electron effects that manifest themselves in the processes of the ionization and excitation of atomic systems. It is these calculations that have stimulated numerous theoretical and experimental studies of many-particle processes in various research centers and laboratories.

The theoretical group created by Prof. Miron Ya. Amusia has developed and described the concepts of a computational experiment that is currently considered to be a new methodology and technology of scientific research ^{[14][15]}[14,15]. The implementation of this concept makes it possible to use the capabilities of computers in combination with existing traditional research methods, creating a new style of research work that combines the work of theoreticians, experimenters and computer programmers.

- Wave functions of atoms in the ground and excited states in the HF and Hartree–Fock–Dirac approximations.
- Amplitudes and cross sections of photoionization (including the strength of oscillators of discrete transitions) of atoms with filled and half-filled shells while taking the interaction between electrons of one, two or more shells into account.
- Atom polarizability.
- Characteristics of the angular distribution of photoelectrons and secondary electrons both in the dipole and outside the dipole approximation.
- Parameters of the spin polarization of photoelectrons.
- Scattering cross sections of fast electrons through the generalized oscillator strengths, taking the influence of electrons in one, two or more shells into account.
- Angular distribution of secondary electrons arising from the inelastic scattering of fast particles on atoms.
- Phases and cross sections of the elastic and inelastic scattering of particles (electrons, positrons, and mesons) of low and medium energies on atoms.
- Photoabsorption cross sections with allowance for the decay of vacancies and the inelastic scattering of a photoelectron.
- Cross sections of the ionization and excitation of an atom by electron impact.
- Probabilities of the single-electron and double Auger decay of vacancies in atoms.
- Probabilities of the one-photon decay of one- and two-hole states.
- Characteristics of bremsstrahlung of high and intermediate energy incident particles.
- Characteristics of capture of mu-mesons by atoms.
- Amplitudes, photoionization cross sections, and angular anisotropy parameters of endohedral atoms and the decay of vacancies in such atoms.
- Characteristics of the inelastic scattering of fast electrons on endohedral atoms.
- Characteristics of photoabsorption processes in negative and positive ions.