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The role of repulsive forces in determining molecular conformation and in the interpretation of crystal packing is analyzed by theoretical methods, including ab initio energy calculations, atom–atom potentials and Monte Carlo methods, and above all, PIXEL energy calculations which explicitly account for the subdivision of total intermolecular energies into various contributions. Classes of molecular systems are analyzed and repulsion terms are shown to correlate broadly with chemical constitution. Intermolecular contacts or molecular arrangements that are often thought to be attractive, or stabilizing, and even structure-determining on the basis of qualitative inspection, when analyzed by quantitative energetic calculations appear to involve repulsive interactions. A study of the response of crystal structures to external pressure, using pressure-dependent X-ray diffraction studies extracted from crystallographic databases, gives an estimate of bulk moduli and compressibility, also broadly related to the steepness of the repulsion wall in crystals of different classes of compounds. Finally, the frequencies of hydrogen bond formation over a very large database of crystal structures of multi-functional organic compounds shows that this bond, whose formation is usually thought to be universal and highly predictable, is useful in crystal engineering exercises over classes of compounds with pre-designed attachment points, but is not always a reliable predictor of intermolecular structuring in crystals of complex organic molecules when competition between several bonding modes arises.

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