Ab initio phasing based on topological restraints: automated determination of the space group and the number of molecules in the unit cell
Direct phasing starting from low resolution is complementary to conventional phasing methods and may be efficient when these techniques fail or are inapplicable. The approach described here uses a single set of structure factor magnitudes and the expected number of ‘blobs’ visualized in the corresponding Fourier maps (each of the ‘blobs’ stands for a molecule or a molecular domain). The method discriminates between the correct and incorrect assignment of this number, therefore providing a way to determine the number of molecules in the unit cell as well as to select the correct space group in ambiguous cases.
L. Urzhumtseva, N. Lunina, A. Fokine, J.-P. Samama, V. Y. Lunin and A. Urzhumtsev
![[electron density]](https://www.iucr.org/__data/assets/image/0007/18844/12-3_Page_06_Image_0018.jpg)
An attempt of phasing in an incorrect space group results in a nonsense image of the structure (right); compare with the image obtained for the correct choice of the space group (left).
