Complete Cross Validation in Crystallography
History of Cross Validation: Rfree
In the 1990s it became clear that the R1-value is not a good quality measure for crystal structures, in particular for macromolecular structures which suffer from a low data to parameter ratio [1, and references therein].
Axel Brunger introduced the concept of cross-validation into protein crystallography, and nowadays, the usage and importance Rfree is generally accepted for macromolecular structures.
As A Brunger already pointed out that the Rfree is actually a compromise between complete cross-validation and feasibility given the computer power at that time.
Complete Cross Validation: Rcomplete
The speed of modern computers allow to carry out proper, complete cross validation and therefore get a fully valid, unbiased R-value for crystal structures from all reflections (hence the term 'complete cross validation').
The latest version of SHELXL prints the required numbers that are necessary for the calculation of Rcomplete. Together with my program crossflaghkl (available upon request) it is easy to calculate Rcomplete at any stage during model refinement.
For more information and uses, please check out my latest talk Modern Cross Validation in Crystallography on this topic.
This is work in progress. Please contact me for more information.
- G. J. Kleywegt, T. A. Jones, Where freedom is given, liberties are taken, Structure (1995), volume 3, 535-540
- A. T. Brünger, Cross-Validation in Crystallography, Meth. Enzymol. (1990), volume 343, 366-396
Last modified: Jul 17, 2014 11:24