Section 25.2.2.5.7. Multi-crystal averaging

The multi-crystal averaging calculation in DMMULTI is equivalent to several single-crystal averaging calculations running simultaneously, with the exception that during the averaging step, the molecule density is averaged across every copy in every crystal form. This average is weighted by the mean figure of merit of each crystal form; this allows the inclusion of unphased crystal forms, since in the first cycle they will have zero weight and therefore not disrupt the phasing that is already present. In subsequent cycles, the unphased form contains phase information from the back-transformed density.

This technique can be extremely useful, since adding a new crystal form usually provides considerably more phase information than adding a new derivative if the cross-rotation and translation functions can be solved.

In the multi-crystal case, averaging is performed using a two-step approach, first building an averaged molecule from all the copies in all crystal forms, then replacing the density in each crystal form with the averaged values. This approach is computationally more efficient when there are many copies of the molecule.

The conceptual flow chart of simultaneous density-modification calculations across multiple crystal forms is shown in Fig. 25.2.2.3(a); in practice, this scheme is implemented using a single process and looping over every crystal form at each stage (Fig. 25.2.2.3b). Maps are reconstructed from a large data object containing all the reflection data in every crystal form. Averaging is performed using a second data object containing maps of each averaging domain. By this means, an arbitrary number of domains may be averaged across an arbitrary number of crystal forms.

Multi-crystal averaging has been particularly successful in solving structures from very weak initial phasing, since the data redundancy is usually higher than for single-crystal problems.