Recent developments in crystal structure prediction, in particular, the powerful evolutionary algorithm USPEX [1,2], enable reliable prediction of stable compounds formed by given elements. At normal conditions such calculations produce the well-known stable compounds: e.g., NaCl as the only compound of Na and Cl, or MgO as the only stable compound of Mg and O. At high pressures and in low-dimensional materials, unexpected phenomena have been predicted-then experimentally verified. I will discuss several recent examples: 1. Discovery of two new stable high-pressure compounds of helium, Na2He and Na2HeO (Na2He has been synthesized experimentally) [3]. This discovery has implications for both fundamental chemistry and planetary sciences. 2. Formation of new stable sodium chlorides: Na3Cl, Na2Cl, Na3Cl2, NaCl3, NaCl7 [4], Na4Cl3 [5], and a large number of new stable potassium chlorides [6]. These predictions were verified experimentally [5,6] and are still not fully understood. 3. New stable magnesium oxides: Mg3O2 and MgO2 [7] and MgO3 [8], and silicon oxides SiO and SiO3 [8]. Among these predictions, stability of MgO2 has already been experimentally confirmed [9]. These predictions may have implications for planetary chemistry. 4. USPEX-based prediction of the Cui group [10] and experimental verification of Eremets group [11] of a new high-temperature superconductor-cubic H3S. This discovery opens new hopes for room-temperature superconductivity. 5. Prediction [12] that dominant silicon oxide nanoparticles at normal conditions (ambient P-T, and normal air) will be oxygen-enriched and magnetic: e.g. Si7O19. This may explain well-documented carcinogenic activity of fine silica dust. Future avenues for explanation and generalization of these phenomena will be discussed. 1] Oganov A.R. et al, J.

Non-covalent interactions (NCI) define the rules underlying crystallisation, self-assembly and drug-receptor docking processes. A novel NCI descriptor, based on the reduced electron density gradient (RDG), that enables easy visualisation of the zones of the electron density (ED) involved in either the supposedly attractive (dispersive, hydrogen bonding) or allegedly repulsive (steric) intermolecular interactions, was recently developed by Johnson et al. Here, it is applied for the first time to EDs derived from single-crystal X-ray diffraction data. A computer code handling both experimental and ab initio EDs in the RDG-NCI perspective was purposely written. Three cases spanning a wide range of NCI classes were analysed: 1) benzene, as the prototype of stacking and weak CH···π interactions; 2) austdiol, a heavily functionalised fungal metabolite with a complex hydrogen-bonding network; 3) two polymorphs of the heteroatom-rich anti-ulcer drug famotidine, with van der Waals and hydrogen-bond contacts between N- and S-containing groups. Even when applied to experimental EDs, the RDG index is a valuable NCI descriptor that can highlight their different nature and strength and provide results of comparable quality to ab initio approaches. Combining the RDG-NCI study with Bader's ED approach was a key step forward, as the RDG index can depict inherently delocalised interactions in terms of extended and flat RDG isosurfaces, in contrast to the bond path analysis, which is often bounded to a too localised and possibly discontinuous (yes/no) description. Conversely, the topological tool can provide quantitative insight into the simple, qualitative NCI picture offered by the RDG index. Hopefully, this study may pave the way to a deeper analysis of weak interactions in proteins using structural and ED information from experiment.

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