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Abstract
The determination of macromolecular structure by crystallography involves fitting
atomic models to the observed diffraction data. The traditional measure of the quality
of this fit, and presumably the accuracy of the model, is the R value. Despite stereochemical
restraints, it is possible to overfit or 'misfit' the diffraction data: an incorrect
model can be refined to fairly good R values as several recent examples have shown.
Here I propose a reliable and unbiased indicator of the accuracy of such models. By
analogy with the cross-validation method of testing statistical models I define a
statistical quantity (R(free) (T) that measures the agreement between observed and
computed structure factor amplitudes for a 'test' set of reflections that is omitted
in the modelling and refinement process. As examples show, there is a high correlation
between R(free) (T) and the accuracy of the atomic model phases. This is useful because
experimental phase information is usually inaccurate, incomplete or unavailable. I
expect that R(free) (T) will provide a measure of the information content of recently
proposed models of thermal motion and disorder, time-averaging and bulk solvent.