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The results of LCAO DFT calculations of lattice parameters, cohesive energy and bulk modulus of the crystalline uranium nitrides UN, U2N3 and UN2 are presented and discussed. The LCAO computer codes Gaussian03 and Crystal06 are applied. The calculations are made with the uranium atom relativistic effective small core potential by Stuttgart-Cologne group (60 electrons in the core). The calculations include the U atom basis set optimization. Powell, Hooke-Jeeves, conjugated gradient and Box methods are implemented in the author's optimization package, being external to the codes for molecular and periodic calculations. The basis set optimization in LCAO calculations improves the agreement of the lattice parameter and bulk modulus of UN crystal with the experimental data, the change of the cohesive energy due to the optimization is small. The mixed metallic-covalent chemical bonding is found both in LCAO calculations of UN and U2N3 crystals; UN2 crystal has the semiconducting nature.

Hybrid HF-DFT LCAO simulations of (001) surface properties and water adsorption on cubic SrTiO3, SrZrO3, and SrHfO3 perovskites are performed in a single-slab model framework. The optimized atomic structures and water adsorption energies have been calculated for a single water molecule per the surface unit cell. The possibility of the water molecular dissociation was investigated. Basing on the experimental data and results of the ab initio calculations the new interatomic potentials have been developed to describe the bulk and surface properties of the binary and ternary titanium and zirconium oxides. The proposed force-field takes into account the polarization effects via the shell model. The force-field suggested was used in the molecular mechanics calculations with the extended unit cells to study the possible surface reconstruction upon relaxation and hydroxylation of cubic perovskites.

The electronic structures of four well-known modifications of crystalline SrZrO3 with different symmetries, namely, the cubic (Pm3m), tetragonal (I4/mcm), and two orthorhombic (Cmcm, Pbnm) modifications, are calculated in the framework of the density-functional theory in the basis set of the linear combination of atomic orbitals (LCAO). A comparative analysis of the electronic properties of the crystals under consideration is performed on the basis of the calculated band structures and densities of states (the total densities of states and the densities of states projected onto the atomic states). The calculated relative stabilities of the different modifications are in good agreement with the experimental data on the phase transitions in the SrZrO3 crystal: the low-temperature modifications with lower symmetry are more stable. The ionicities of chemical bonding in different modifications of crystalline SrZrO3 are compared by analyzing the Mulliken populations and constructing the localized Wannier functions for the occupied energy bands.

Quantum-chemical calculations on molecular complexes containing ions and molecules of various solvents are used widely for studying the behavior of electrolyte solutions and for elucidating the properties of structures which form in them. Both nonempirical (ab initio) [1-14] and semiempirical methods [15-21] are used for these purposes. Ab initio methods, within the limits of the self-consistent field Hartree--Fock (SCF HF) approximation, give correct results for the equilibrium distances and formation energies of molecular complexes. For example, the formation energies of hydrates of Li +, Na +, K +, CI-, and Fcalculated in [1-3] agreed with experimental data [22] within limits of 1-2 kcal/mole. Numerous calculations of hydrogen bonds [9-14] also confirm the applicability of the HF approximation for studying interactions between polar molecules at distances up to 5 ~.

The molecular cluster approach to the problem of small-radius impurity centres in solids is discussed. It is shown that the molecular orbitals obtained for the cluster of host atoms can be connected with the crystalline states only for clusters having the form of a large unit cell. In this case the molecular orbitals of the cluster have the sense of Wannier functions, the appropriate one-electron energies the sense of mean values of the band energies in the large unit cell scheme of the crystalline state classification. It is found that the cluster model is realistic for solids with narrow (1 to 5 eV) energy bands. The convergence of the results with increasing cluster size is shown for lithium chloride ([LiCl6]5− [Li14Cl13]+, and [Li18, Cl18]0 are considered), the molecular cluster calculations are performed for some alkali halides and the results are compared with those obtained using band structure calculations. [Russian Language Ignored].