R factors in X-ray fiber diffraction. Pt. 4

The largest likely R factor is useful for evaluating the significance of R factors obtained in structure determinations. Numerical expressions have been derived previously for calculating largest likely R factors in fiber diffraction analyses. Analytical approximations to largest likely R factors (R) in fiber diffraction are derived here that show the dependence on resolution (ρ), helix symmetry (uυ) and molecular radius (r). The simplest approximation is R∝(u/rρ)1/2 which represents the overall behavior of R factors reasonably well. More are applied to various structures and the dependence on different structural parameters is examined. These results provide insight into the behavior of R factors in fiber diffraction and may be useful in further analysis. (orig.)