International
Tables for Crystallography Volume F Crystallography of biological macromolecules Edited by E. Arnold, D. M. Himmel and M. G. Rossmann © International Union of Crystallography 2012 
International Tables for Crystallography (2012). Vol. F, ch. 16.1, pp. 421422
Section 16.1.9.3. Recognizing solutions
G. M. Sheldrick,^{a} C. J. Gilmore,^{b} H. A. Hauptman,^{c}^{‡} C. M. Weeks,^{c}^{*} R. Miller^{c} and I. Usón^{d}
^{a}Lehrstuhl für Strukturchemie, GeorgAugustUniversität Göttingen, Tammannstrasse 4, D37077 Göttingen, Germany,^{b}Department of Chemistry, University of Glasgow, Glasgow G12 8QQ, UK,^{c}Hauptman–Woodward Medical Research Institute, Inc., 700 Ellicott Street, Buffalo, NY 14203–1102, USA, and ^{d}Institució Catalana de Recerca i Estudis Avançats at IBMBCSIC, Barcelona Science Park. Baldiri Reixach 15, 08028 Barcelona, Spain 
On account of the intensive nature of the computations involved, SnB and SHELXD are designed to run unattended for long periods while also providing ways for the user to check the status of jobs in progress. The progress of current SnB jobs can be followed by monitoring a figureofmerit histogram for the trial structures that have been processed (Fig. 16.1.9.2). A clear bimodal distribution of figureofmerit values is a strong indication that a solution has, in fact, been found. However, not all solutions are so obvious, and it sometimes pays to inspect the best trial even when the histogram is unimodal. The course of a typical solution as a function of SnB cycle is contrasted with that of a nonsolution in Fig. 16.1.9.3. Minimalfunction values for a solution usually decrease abruptly over the course of just a few cycles, and a tool is provided within SnB that allows the user to visually inspect the trace of minimalfunction values for the best trial completed so far. Fig. 16.1.9.3 shows that the abrupt decrease in minimalfunction values corresponds to a simultaneous abrupt increase in the number of peaks close to true atomic positions. In this example, a second abrupt increase in correct peaks occurs when Fourier refinement is started.

A histogram of figureofmerit values (minimal function) for 378 scorpion toxin II trials. This bimodal histogram suggests that ten trials are solutions. 
Since the correlation coefficient is a relatively absolute figure of merit (given atomic resolution, values greater than 65% almost invariably correspond to correct solutions), it is usually clear when SHELXD has solved a structure, although when the data do not extend to atomic resolution the CC values are less informative, and for a substructure they depend strongly on the data quality.