This is an archive copy of the IUCr web site dating from 2008. For current content please visit https://www.iucr.org.

... molecules¹

From now on, the term molecule denotes a molecule proper, or any other chemical entity also recognizable in the gas phase (a helium atom, an Na⁺ or SO₄^2- ion, an Fe₂(CO)₉ complex). In general, it can be said that a molecule is a distinguishable entity when the forces acting within it are much stronger than the forces acting on it in the crystal. Difficulties arise with infinite strings or layers; diamond and NaCl crystals are examples of three-dimensionally infinite systems where the term molecule is meaningless. Also, whenever organic compounds are mentioned in the text, one should read organic and organometallic compounds.

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... symmetry²

The term crystal symmetry refers to microscopic relationships between molecules or parts of molecules, and not to macroscopic morphology.

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... coefficient³

The packing coefficient is the ratio of volume occupied by the molecules in the cell to the volume of the cell. Molecular volumes can be calculated in a number of ways; the simplest ones are described by Kitaigorodski^[5], and others by Gavezzotti^[7].

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... elements⁴

A symmetry operation moves or maps isometrically one point to another. A symmetry element is a geometric object, viz a point, a line or a plane, assigned specifically to a set containing one or several symmetry operations. The distinction between a symmetry operation and a symmetry element is explained in detail in the opening chapters of International Tables for Crystallography Vol. A^[10].

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