Section 16.1.8.5. Expansion to P1

The results shown in Table 16.1.8.4 and Fig. 16.1.8.3 indicate that success rates in space group P1 can be anomalously high. This suggests that it might be advantageous to expand all structures to P1 and then to locate the symmetry elements afterwards. However, this is more computationally expensive than performing the whole procedure in the true space group, and in practice such a strategy is only competitive in low-symmetry space groups such as , C2 or (Chang et al., 1997). Expansion to P1 also offers some opportunities for starting from `slightly better than random' phases. One possibility, successfully demonstrated by Sheldrick & Gould (1995), is to use a rotation search for a small fragment (e.g. a short piece of α-helix) to generate many sets of starting phases; after expansion to P1 the translational search usually required for molecular replacement is not needed. Various Patterson superposition minimum functions (Sheldrick & Gould, 1995; Pavelčík, 1994) can also provide an excellent start for phase determination for data expanded to P1. Drendel et al. (1995) were successful in solving small organic structures ab initio by a Fourier recycling method using data expanded to P1 without the use of probability theory.