Improved free R factors for cross-validation of macromolecular structure -importance for real-space refinement

Improvements in free R cross-validation are based on changed scaling proced ures and the use, in map calculation, of estimates of the validation amplit udes which are independent of the actual observed values. The deleterious e ffects of the omitted test data are mitigated by reduction of the test-set size, which is made possible by constraining test and working sets to share the same scaling coefficients, thereby reducing the degrees of freedom and the dependence of free R on data selection. Further improvements come with use of a modified free R factor, R-TA(free). Instead of omitting the valid ation reflections from map calculation, their amplitudes are replaced by th e average of resolution peers that is (nearly) independent of the actual cr oss-validation amplitudes. The improvements are relevant to model building, phase refinement by density modification and especially to real-space refi nement. Although for Peal data at about 3 Angstrom resolution, free R facto rs of about 0.25 are affected little, the precision of the structure is imp roved by about 0.1 Angstrom. Tests with simulated data show that with good agreement between observed and calculated amplitudes (as in very high resol ution studies or simulated refinement tests), free R factors can be improve d by factors greater than two.