My recent scientific activities are concerned with the theory of defects in solids, with ab initio and semiempirical calculations of the electronic structure of perfect insulating crystals, surfaces and defects, with mechanisms of defect creation under irradiation, defect diffusion, and recombination, with the kinetics of bimolecular and tunnelling reactions. That is, I study problems related to theoretical condensed matter physics and radiation physics of insulating solids, as well to theoretical quantum chemistry of non-metallic crystals with special emphasis on the materials science aspects. In particular, I have developed a phenomenological theory of diffusion- controlled bimolecular reactions in solids and liquids incorporating electron tunnelling, studied self- organisation effects in chemical reactions, established the essential role of electron tunnelling and elastic interaction of radiation defects in insulating solids during their accumulation under irradiation. I have performed ab initio calculations of pure and CO-, SiO- and Ag-covered oxide crystalline surfaces as well as semiempirical INDO calculations of the electronic structure of numerous point defects and relevant processes in ionic solids.