Quantum chemical methods in charge density studies from X-ray diffraction data

Single-crystal X-ray diffraction studies are among the best experimental methods for elucidating the structure of chemical compounds. Recently, their focus has been shifted towards extracting information on chemical bonding features and related valuable properties of these compounds from the analysis of charge density distribution obtained in high-resolution X-ray diffraction experiments; the latter are possible only for a limited number of well-ordered crystals of small molecules. In this review, the hybrid approaches are described that introduce quantum chemical methods into the refinement of X-ray diffraction data. Their use significantly extends the range of systems suitable for charge density studies (such as polypeptides, metal-organic frameworks, inclusion compounds and others) and the scope of problems that they solve, viz., from protein structure refinement to determination of thermodynamic or other wave function-derived properties of crystals. The potential of these hybrid approaches and prospects for their future applications are discussed.