Section 25.2.5.3. Applicability and requirements

Density-based atom selection for the whole structure is only possible if the X-ray data extend to a resolution where atomic positions can be estimated from the Fourier syntheses with sufficient accuracy for them to refine to the correct position. If the structural model is of reasonable quality, at 2.5 Å or better, at least a part of the solvent structure or a small missing or badly placed part of the protein can be located. This provides indirect improvement of the whole structure. For automated model rebuilding, or for refining poor molecular-replacement solutions, higher resolution is essential. The general requirement is that the number of X-ray reflections should be at least six to eight times higher than the number of atoms in the model, which roughly corresponds to a resolution of 2.3 Å for a crystal with 50% solvent. However, the method can work at lower resolution or fail with a higher one, depending less on the quality of the initial phases and more on the internal quality of the data and on the inherent disorder of the molecule.

The X-ray data should be complete. If strong low-resolution data (e.g. 4 to 10 Å) are systematically missing, e.g. due to detector saturation, the electron density even for good models is often discontinuous. Because ARP involves updating on the basis of density maps, such discontinuity will lead to incorrect interpretation of the density and slow convergence or even uninterpretable output.