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International
Tables for Crystallography Volume F Crystallography of biological macromolecules Edited by M. G. Rossmann and E. Arnold © International Union of Crystallography 2006 |
International Tables for Crystallography (2006). Vol. F. ch. 25.2, p. 701
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The scaled data are sorted into bins according to d spacing, and a three-term polynomial is fitted to the mean values of ![[|F^{2}_{\rm obs} - F^{2}_{\rm calc}|]](/teximages/fach25o2/fach25o2fi50.svg)
as a function of resolution. For each reflection, a unimodal phase probability distribution is constructed using a modification (Bricogne, 1976![[link]](/graphics/greenarr.gif)
) of the Sim (1959) weighting scheme via ![[P(\varphi_{P}) = k \exp \left[{2F_{\rm obs} F_{\rm calc} \cos (\varphi_{P} - \varphi_{\rm calc}) \over \langle | F_{\rm obs}^{2} - F_{\rm calc}^{2}|\rangle}\right], \eqno(25.2.1.29)]](/teximages/fach25o2/fach25o2fd29.svg)
where the average in the appropriate resolution range is determined from the polynomial. This distribution is cast in the A, B, C, D form with ![[\eqalignno{ A &= W \cos (\varphi_{\rm calc}), &\cr B &= W \sin (\varphi_{\rm calc}), &\cr C &= 0 &\cr D &= 0\;\;{\rm and}&\cr W &= {2F_{\rm obs} F_{\rm calc} \over \langle |F_{\rm obs}^{2} - F_{\rm calc}^{2}|\rangle}. &(25.2.1.30)\cr}]](/teximages/fach25o2/fach25o2fd30.svg)
Phase combination with the anchor set then proceeds according to equation (25.2.1.10), and the combined distributions are integrated to give a new phase and figure of merit for each reflection.
References
Bricogne, G. (1976). Methods and programs for direct-space exploitation of geometric redundancies. Acta Cryst. A32, 832–846.Google Scholar
Sim, G. A. (1959). The distribution of phase angles for structures containing heavy atoms. II. A modification of the normal heavy-atom method for non-centrosymmetrical structures. Acta Cryst. 12, 813–814.Google Scholar
