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Theoretical concepts of the positron annihilation process in structural materials of nuclear reactors, taking into account the peculiarities of their electronic structure, have been developed. The Tao - Eldrup model, which allows to analytically calculate the lifetime of a positron in a spherically symmetric potential well, has been modified for the case of a potential well of finite height, in order to expand the limits of the model's application. The dependence of the positron lifetime on the height and width of the potential well, which occurs at the point defects, was determined. The results obtained within the framework of the modified model provide important information for the analysis of positron lifetime spectra in irradiated materials and data for the verification of quantitative calculations by the method of density functional theory. [ABSTRACT FROM AUTHOR]

Cadmium sulfide (CdS) crystal is a representative of AIIBVI crystal group and exhibits typical semiconductor behavior. CdS crystal can exist in either zinc-blende or hexagonal (wurtzite) structures under normal conditions, depending on the growth conditions. These crystals remain an important research Field because of their wide application in various Fields of optoelectronic devices. From this point of view, the study of structural properties and the dynamics of electronic spectra of CdS crystal under the action of external pressures is of fundamental interest for studying the process of stabilization in compounds. We have investigated the effect of pressure on the electronic band structure of CdS crystal using the density functional theory. In this approach, the generalized gradient approximation (GGA) and the Purdue-Burke-Ernzerh of (PBEsol) parameterization were used for the exchange-correlation potential calculation. The ground-state properties were determined for the bulk material (CdS) in zinc blende structure. Structural and electronic properties were studied in the range of hydrostatic pressures between 0 and 90 GPa. The electron band-energy structure and density of states were calculated at different pressures. The equilibrium structural parameters, bulk modules and bandgaps were calculated and compared with the available experimental data and other theoretical results. The effect of pressure on the energy gap, total energy, and lattice parameters of CdS crystal has been determined and compared with other known data. [ABSTRACT FROM AUTHOR]

In recent years, inorganic cesium lead bromide (CsPbBr3) perovskite has been widely studied due to its potential spplication in light-emitting devices and solar cells. In this work, within the density functional theory framework, we performed a study of structural and electronic properties of temperature-dependent phases of the CsPbBr3 crystal, in particular, the cubic, tetragonal, and orthorhombic phases using different approximations for the exchange-correlation functional. The analysis of structural parameters shows that the lattice constants and the volume of the unit cell change when the CsPbBr3 crystal changes from the cubic phase to the tetragonal and orthorhombic structures. A good agreement between theoretical and experimental results is obtained using GGA(PBEsol) and GGA(PBEsol)+U approximations. The Mulliken population analysis indicates that between Cs and Br atoms there is a stronger ionic bonding and at the same time there is a stronger covalent bonding between Br and Pb atoms. The electronic structure of CsPbBr3 perovskite was investigated by estimating the change in the electronic properties when included in the calculations of spin-orbit coupling (SOC). The results of the calculations of the band-energy structure showed that the cubic, tetragonal, and orthorhombic crystalline phases of perovskite are semiconductors and have direct bandgaps. Also, we found that the bandgap changes with a change in the phase structure. The obtained values of the bandgap for all crystalline phases of CsPbBr3 perovskite are in good agreement with the previously obtained theoretical calculations as well as experimental data. We established that the tilting angle has a crucial effect on the bandgap width of the tetragonal and orthorhombic phases. The results of the obtained first-principle calculations show a wide temperature range of the possible use of CsPbBr3 perovskite. [ABSTRACT FROM AUTHOR]

Context. The paper presents a computational scheme and a number of techniques for increasing the efficiency of mathematical modeling of the energy characteristics of metal clusters with vacancies. The object of the research is the process of calculating wave functions and finding eigenvalues using the Numerov and the shooting methods. Objective. The objective of the research is to improve the quality of the calculation of the wave functions of electrons in a metal cluster by using optimally selected methods and techniques in the computational scheme, to ensure the adequacy, stability and cost-effectiveness of self-consistent calculations of the energy characteristics of metal clusters by limiting the change in the electrostatic potential within a single iteration, choosing the optimal integration step and data presentation format. Method. At the modeling stage, to increase the efficiency, the electron Density Functional Theory was used in conjunction with the Kohn-Sham version for the stabilized jellium model taking into account the local density approximation for calculating the energy characteristics of nanoclusters. At the simulation stage, the one-electron wave function was calculated by “stitching” it from two parts at an empirically selected point with normalization before and after the procedure. A number of techniques have been developed to improve the quality of the simulation: calculating the optimal step, limiting changes in the electrostatic profile, managing the resulting data arrays, etc. For calculations on a supercomputer, the distribution between the flows was carried out. Results. At the modeling stage, economic models of the metal sphere with a vacancy in the center were developed. For the simulation stage, a method of stable two-sided calculation of the wave function using the shooting and the Numerov methods with the optimal step has been developed. The full computational scheme for the simulation is implemented in C ++ for calculation on a PC and on a supercomputer. The simulation results were compared with the \emph{ab-initio} calculation data and experimental data for Cs, Rb, K, Na, Li, Mg, and Al clusters with and without vacancies (calculation error < 15\ Conclusions. The developed computational scheme and modeling technique allow increasing the simulation efficiency and obtaining adequate energy characteristics of metal spherical nanoclusters with and without vacancies. Further research might focus on the modification of models and simulation techniques for the study of layered nanoscale systems. [ABSTRACT FROM AUTHOR]

Studying interaction of zinc (II) with silica and alumina-silica surfaces is important for several reasons. From a practical point of view, it is carried out in the framework of the creation of a medicinal product, the ingredients of which are, in particular, silica(alumina)containing sorbent and zinc compound; from a theoretical point of view, it is a search for the adsorption patterns on the mentioned surfaces in a series of d-elements with a close electronic structure (zinc, copper, nickel, cadmium, etc.). In addition, it is advisable to supplement the experimental adsorption data with the results of quantum chemical simulation of the structure and formation energy of zinc (II) complexes with active surface centers. Adsorption of Zn2+ ions from a neutral aqueous solution on silica and alumina-silica materials was studied by means of the method of adsorption isotherm. The specific surface of the materials was determined by method of thermal desorption of argon. Quantum chemical analysis of the structure and energy of formation of the hydrated zinc sulfate ion pair on silica and alumina-silica surfaces was performed by means of Hartree-Fock-Roothaan method with extended basis set 6-31++G(d,p). According to the magnitude of adsorption (A) of zinc, the test materials are divided into three groups: 1) enterosgel, kaolin, nanosilica A-300 and polymethylsiloxane, for which Аmax is from 0.15 to 0.3 mg/g; 2) Syloid® 244 FP, KG-60 and KG-40, aminopropylaerosil, smectite (Аmax from 1.2 to 12.0 mg/g); 3) zeolites NaA-TK-1173 and NaX (Аmax more than 30 mg/g). It is concluded that the high adsorption activity of zinc(II) is primarily due to the presence of developed porous structure. The most probable places of interaction between hydrated zinc ions and hydroxyl group of silica and alumina-silica surfaces are determined by means of quantum chemical simulation. The pKMe values have been calculated of ion exchange of silica surface when interacting with Zn2+ cations. [ABSTRACT FROM AUTHOR]

The purpose of this work is to determine the influence of spin-orbital interaction on the electronic energy spectrum of CsPbBr3 and CsPbI3 crystals. The goal was achieved in the following sequence. At the first stage, partial densities of electronic states were calculated and a symmetrical analysis of the electronic structure of the crystals was performed. The calculations were made from the first principles in the formalism of the density functional theory in the basis of the projector augmented waves (PAW). The semilocal exchange-correlation potential derived within the generalized gradient approximation (GGA) was used in the calculations. At the second stage, the electronic energy bands were obtained without taking into account the spin-orbital interaction. The third stage consisted in obtaining the electron energy spectrum of the crystals including the spin-orbital interaction in the calculation scheme. A significant influence of the spin-orbit interaction on the values of the interband gaps ∆E(k) of both crystals has been revealed. In particular, it has been found that the spin-orbit interaction reduces interband gaps in the CsPbBr3 crystal at the points Γ, X, M and R of the first Brillouin zone by 0.17, 0.22, 0.66 and 1.01 eV, respectively. In the CsPbI3 crystal, the reduction of interband gaps at the same points was found to be 0.31, 0.25, 0.73 and 1.08 eV, respectively. Such significant dependence of the parameter ∆E(k) on the momentum indicates a strong non-locality of the spin-orbital interaction operator of CsPbBr3 and CsPbI3 crystals. Consequently, it is not justified to compare the calculated parameters of the electron energy spectrum obtained without the spin-orbital interaction for CsPbBr3 and CsPbI3 crystals with the experiment. Neglecting the spin-orbit interaction can lead to significant errors in the calculation of the optical constants of crystals containing heavy elements. The obtained parameters ∆E(k) can be used to correct the values of the interband gaps of CsPbBr3 and CsPbI3 crystals found without taking into account the spin-orbital interaction on the basis of other theoretical methods. [ABSTRACT FROM AUTHOR]

Density functional theory method with functional B3LYP and 6-31G (d,p) basis set has been used to examine the possible structures of acridine orange fragments resulting from laser irradiation of dye adsorbed on the surface of thermoexfoliated graphite under mass spectrometric experiment. The structures and thermodynamic parameters of these fragments have been found as well as the order of their formation. [ABSTRACT FROM AUTHOR]

A comparative analysis has been performed on the applicability of quantum chemical methods of different complicability to describe the structure and properties of the clean Si(001) surface, and of that containing a few atoms of germanium. It has been found that the density functional theory within the cluster approximation involving the exchange-correlation functional B3LYP and 6-31 G** basis set is the most rational from the point of view of reproducibility of experimentally found results and considering the moderate cost of computing time and computer resources. It is just this approximation of quantum chemical calculations that properly reproduces the lengths of >Si-Si<, >Si-Ge<, and >Ge-Ge< surface dimers bonds as well as the experimentally justified fact of their buckling. [ABSTRACT FROM AUTHOR]