6.6.2 Symmetry-Equivalence Colouring

If all atoms are coloured by Symmetry equivalence (see Section 6.2.2), each different molecule (or ion) in the crystal chemical unit is assigned a different colour (the crystal chemical unit is the same as the asymmetric unit in the majority of structures (see Crystal Chemical Unit)).

When a packing diagram is then constructed, any given molecule is assigned the same colour as that of the molecule in the crystal chemical unit to which it is related by crystallographic symmetry.

This means that molecules of the same colour are crystallographically (and therefore chemically and geometrically) identical to each other.

For example:

If the crystal chemical unit of a crystal contains only one molecule, and a packing diagram of several unit cells is constructed, all molecules will have the same colour, e.g.

If the crystal chemical unit contains two chemically different molecules or ions, each will be assigned a different colour and the molecules in the packing diagram will have two colours, e.g.

If the crystal chemical unit contains two molecules which are chemically identical but not related by crystallographic symmetry, each will be coloured differently and, again, the packing diagram will contain molecules of two colours, e.g.