Here a counter example is presented: The Rietveld refinement of the X-ray powder pattern of γ-quinacridone (γ-1) with the crystal structure of β-quinacridone (β-1) gives a quite smooth difference curve, even though the crystal
structures as well as the powder patterns (Fig. 1) are completely different. The crystal structure from single data shows that in γ-1 molecular chains in a criss-cross pattern are formed; each molecule forms hydrogen bonds to 2 neighbouring molecules. In β-1 the molecules are arranged in chains; each molecule exhibits hydrogen bonds to 4 neighbouring molecules.

The crystal structure from Rietveld refinement looks reasonable in terms of molecular conformation, molecular packing and intermolecular hydrogen bonds. But neither the lattice parameters, nor the molecular packing, nor the conformation of the molecules have any similarity with the actual structure. This example shows, that a successful Rietveld refinement is not always a final proof for the correctness of a crystal structure; in special cases the resulting crystal structure may still be wrong.

Fig. 1: Powder patterns of β-1 (top) and γ-1 (bottom).

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