The approach to the analysis of the crystal structure of organic compounds based of the intermolecular interaction energies has been suggested. This approach creates the necessary prerequisites for development of a lot of directions for study:
-investigation of the influence on a type of crystal packing of the organic molecule fragments (substituents, type of hybridization of some carbon atoms, etc.);
-comparative analysis of the role of intermolecular interactions in the formation of crystal structure;
-disengagement of the levels of organization for molecular crystals: building unit (molecule or molecular complex), basic structural motif (colomn or layer) and 3D crystal structure;
-ranking of intermolecular interactions in crystals;
-comprehensive study of the relationship between crystal structure and mechanic or optic properties of molecular crystals;
-study of the polymorphism of molecular crystals based on the energy of intermolecular interactions.
Application of the known methods of wavefunction analysis and developing of the new ones. The Excited State Structural Analysis scheme, developed formerly for the TD DFT, was generalized to the spin-flip method SF CIS. Now it can handle degenerated excited states, like ones of NV-defect in nanodiamonds (red balls marks localization of lower electron excitation).
Investigation of carbocyclic and heterocyclic aromatic rings using ab initio quantum chemical and Car-Parinello molecular dynamic methods confirms that all of them demonstrate very high conformational flexibility. It increases dramatically with increasing the temperature from 0 K to 298 K, parallel with decreasing of degree of aromaticity. That difference is highest for highly symmetrical molecules like benzene due to entropy factor. E.g., at the room temperature, mean value of HOMA aromaticity index is 0.85 for benzene and 0.89 for pyrimidine, and the percent of non-planar molecules is 93% and 70%, respectively. 
Analysis of substituted aromatic species demonstrates strong dependence of distribution of π-electrons subject to both intramolecular and intermolecular interactions. Presence of intramolecular resonance-assisted hydrogen bond decreases aromaticity of benzene ring, and cooperative effect of such interactions could reduce it to 20% of unsubstituted molecule. Influence of intermolecular interactions is less pronounced, but it was established that push-pull effect in substituted benzenes in the condensed phase is triggered by presence of intermolecular hydrogen bonds supported by non-specific electrostatic interactions.
Complex application of X-ray single crystal and powder diffraction study seems to be a very efficient tool for study new materials and substances for which one of named method may provide only partial and non-complete results. E.g. for LGSO materials (the Lu2SiO5-Gd2SiO5 solid solution system) used as scintillators it was found that (i) two series of solid solutions may be obtained depending on seek used for crystal growth – GSO (sp.gr. P21/c) or LSO type (sp.gr. C2/c); (ii) Rietveld refinement of powder patterns reveals linear dependence of cell dimensions for each serie in accordance with used Gd:Lu ratio in the furnace charge; (iii) single crystal X-ray study shows reliably that in each serie gadolinium atoms preferably occupy a high-coordinated cationic positions whereas lutetium occupy low-coordinated ones. Also using of single crystal structure data allows to provide reliable quantitative phase analyses for such important materials as
• functional materials;
• new inorganic and organic synthetic substances;
• raw natural materials;
• building materials;
• pharmaceutical drugs, substances and polymorphs;
• industrial waste.
Structure solution from the X-ray powder pattern may be applied for the su
Examination of single crystals perfectness using double crystal X-ray spectrometer, X-ray diffraction examination of epitaxial films structure, orientation of crystals for precise cutting.
Fig. 1. a) Grazing incidence X-ray diffraction (GIXRD) analysis allows to define the thickness of thin opaque films. SnTe film on the KCl substrate. Points - experimental data, line - calculated adjustment. Film thickness 280 Å. b) X-ray diffraction pattern of LuTb2AI2Ga3O12:Ce epitaxial film on the YAG substrate obtained by LPE method. Lattice misfit appears to be 0.57%.
Also using of single crystal structure data allows to provide reliable quantitative phase analyses for such important materials as
- functional materials;
- new inorganic and organic synthetic substances;
- raw natural materials;
- building materials;
- pharmaceutical drugs, substances and polymorphs;
- industrial waste.
Structure solution from the X-ray powder pattern may be applied for the examination of single crystals perfectness using double crystal X-ray spectrometer, X-ray diffraction examination of epitaxial films structure, orientation of crystals for precise cutting