Section 25.2.10.3.3. Practical considerations

Since the input files for the direct and Patterson methods in SHELXS and the integrated method in SHELXD are almost identical (usually only one instruction needs to be changed), it is easy to try all three methods for difficult problems. The Patterson map interpretation in SHELXS is a good choice if the heavy atoms have variable occupancies and it is not known how many heavy-atom sites need to be found; the direct-methods approaches work best with equal atoms. In general, the conventional direct methods in SHELXS will tend to perform best in a non-polar space group that does not possess special positions; however, for more than about a dozen sites, only the integrated approach in SHELXD is likely to prove effective; the SHELXD algorithm works best when the number of sites is known. Especially for the MAD method, the quality of the data is decisive; it is essential to collect data with a high redundancy to optimize the signal-to-noise ratio and eliminate outliers. In general, a resolution of 3.5 Å is adequate for the location of heavy-atom sites. At the time of writing, SHELXD does not include facilities for the further calculations necessary to obtain maps. Experience indicates that it is only necessary to refine the B values of the heavy atoms using other programs; their coordinates are already rather precise.

Excellent accounts of the theory of direct and Patterson methods with extensive literature references have been presented in IT B Chapter 2.2 by Giacovazzo (2001) and Chapter 2.3 by Rossmann & Arnold (2001).