Modern quantum chemistry is the quantum theory of structure an dynamics of molecular systems. The development of quantum chemistry for almost 8 decades gave birth to many concepts of modern chemistry and created many calculation technique which are widely used to obtain preliminary as well as high-precision information on molecular properties. In the end of XXth century it becomes the real fundamental base of chemistry and the active tool for the qualitative interpretation of structural features and physical and chemical properties including dynamics of chemical transformations. It turns out to be a very power means to obtaining quantitative results for molecular clusters as well as isolated molecules in free states and in external fields. The computational programs created in the last decades of the XXth century and permanently refined provide the reliable quantitative information on molecular equilibrium configurations, harmonic vibrational frequencies and anharmonic force constants, frequencies and intensities of the first electronic transitions, energies of the formations and potential barriers, the parameters important for interpretation of ESR and NMR spectra, electric and magnetic moments, and many other characteristics of molecular systems. The last two decades turned many quantum chemists to comparatively large molecules especially those with the distinctly pronounced biological activity. Nevertheless, small molecules the calculation results for which can pretend on the highest precision still serve as a strong attractor for the people who deal with methodological as well as applied problems.