Rfree and the rfree ratio. I. Derivation of expected values of cross-validation residuals used in macromolecular least-squares refinement.
The last five years have seen a large increase in the use of cross validation in the refinement of macromolecular structures using X-ray data. In this technique a test set of reflections is set aside from the working set and the progress of the refinement is monitored by the calculation of a free R factor which is based only on the excluded reflections. This paper gives estimates for the ratio of the free R factor to the R factor calculated from the working set for both unrestrained and restrained refinement. It is assumed that both the X-ray and restraint observations have been weighted correctly and that there is no correlation of errors between the test and working sets. It is also shown that the least-squares weights that minimize the variances of the refined parameters, also approximately minimize the free R factor. The estimated free R-factor ratios are compared with those reported for structures in the Protein Data Bank.
Department of Crystallography, Birkbeck College, Malet Street, London WC1E 7HX, England.
I J
IJ
Tickle
R A
RA
Laskowski
D S
DS
Moss
D000465:Algorithms; D018360:Crystallography, X-Ray / Q000379:methods*; D016018:Least-Squares Analysis; D015394:Molecular Structure