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Concluding remarks, and a suggestion

Crystal packing is a fascinating, and at the same time such a complicated phenomenon. The physics of the interaction between molecules is relatively simple, but the rules that determine the ways in which these forces can be satisfied are complex and still obscure. For this reason, crystal packing prediction and control are still far-away goals: there are simply too many spatial possibilities with very nearly the same free energy.

The principles of crystal packing are still largely unknown. No one has a unique and general answer even to the most fundamental question: Why do some substances crystallize readily at ordinary conditions, and others do not? Is there any trend in molecular size, shape, stoichiometry, conformation, polarity, that accounts for the ability to crystallize? And then, more detail; for example, for nonlinear-optics applications, it is important to grow non-centrosymmetric crystals, but no one knows why and when a molecule will adopt an inversion centre in forming its crystal structure.

The problem is being tackled, however. On one side, we have the Cambridge Structural Database (CSD), with an enormous potential for intermolecular information, which can be studied by statistics. On the other side, a number of theoretical techniques can be used; for example, if a reliable intermolecular potential function is available, the packing energies of different crystal structures for the same compound can be calculated and compared^[19]; eventually, a full molecular dynamics simulation may become possible.

In our times, scientific breakthroughs are fostered by large numbers of small, most often unconscious, contributions. The accumulation of basic data plays a key role. But the problem is to look at the right things.

The age of intramolecular structural chemistry is declining for small molecules. There is very little that can be added to the average intramolecular geometrical data collected${}^{[20]}$ by use of the Cambridge Structural Database; anything at variance with these well-established averages is most probably wrong. Long experience has shown that discussing electronic effects in terms of molecular geometry alone is a tricky business. So, if you are an X-ray diffractionist, instead of looking at your molecule, try looking at your crystal. There is plenty to be discovered, at a low cost and with perfectly high confidence, by looking at what molecules do when they interact with each other, and single-crystal X-ray diffraction is still the best technique for this purpose.

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