Section 21.1.7.1.5. Resolution

The nominal resolution limits of a data set are chosen by the crystallographer, usually at the data-processing stage, and ought to reflect the range of Bragg spacings for which useful intensity data have been collected. Unfortunately, owing to the subjective nature of this process, resolution limits cannot be compared meaningfully between data sets processed by different crystallographers. Careful crystallographers will take factors such as shell completeness, redundancy and 〈I/σ(I)〉 into account, whereas others may simply look up the minimum and maximum Bragg spacing of all observed reflections. Bart Hazes (personal communication) has suggested defining the effective resolution of a data set as that resolution at which the number of observed reflections would constitute a 100% complete data set. Alternatively, Vaguine et al. (1999) define the effective (or optical) resolution as the expected minimum distance between two resolved peaks in the electron-density map and calculate this quantity as 2Δ _P/2^1/2, where Δ _P is the width of the origin Patterson peak. One day, hopefully, the term `resolution' will be replaced by an estimate of the information content of data sets. Randy Read (personal communication) has carried out preliminary work along these lines.