Quality of the X-ray Data

The data normally should be of high resolution. Unrestrained xyzB refinement with ARP/wARP at lower resolution can potentially lead to a poorer quality density map. The X-ray data should be complete, especially in the low resolution range (5 Å and lower)¹ If the low resolution strong data are systematically incomplete (e.g. missing or overloaded reflections), the density map, even in the case of a good model, is usually discontinuous and is inconsistent with the model. Because ARP/wARP involves updating on the basis of density maps, such discontinuity can lead to incorrect interpretation of the density and as a result to slow convergence or even non-interpretable maps.

In general, the number of X-ray reflections should be at least 6 times higher than the number of atoms in the model.