Watkin David John, Cooper Richard Ian
Chemical Crystallography Laboratory, Department of Chemistry, University of Oxford, 12 Mansfield Road, Oxford, Oxfordshire OX1 3TA, England.
Acta Crystallogr B Struct Sci Cryst Eng Mater. 2016 Oct 1;72(Pt 5):661-683. doi: 10.1107/S2052520616012890. Epub 2016 Sep 16.
Direct determination of the Flack parameter as part of the structure refinement procedure usually gives different, though similar, values to post-refinement methods. The source of this discrepancy has been probed by analysing a range of data sets taken from the recent literature. Most significantly, it was observed that the directly refined Flack (x) parameter and its standard uncertainty are usually not much influenced by changes in the refinement weighting schemes, but if they are then there are probably problems with the data or model. Post-refinement analyses give Flack parameters strongly influenced by the choice of weights. Weights derived from those used in the main least squares lead to post-refinement estimates of the Flack parameters and their standard uncertainties very similar to those obtained by direct refinement. Weights derived from the variances of the observed structure amplitudes are more appropriate and often yield post-refinement Flack parameters similar to those from direct refinement, but always with lower standard uncertainties. Substantial disagreement between direct and post-refinement determinations are strongly indicative of problems with the data, which may be difficult to identify. Examples drawn from 28 structure determinations are provided showing a range of different underlying problems. It seems likely that post-refinement methods taking into account the slope of the normal probability plot are currently the most robust estimators of absolute structure and should be reported along with the directly refined values.
在结构精修过程中直接测定弗拉克参数，通常会得到与精修后方法不同但相似的值。通过分析近期文献中的一系列数据集，探究了这种差异的来源。最显著的是，观察到直接精修的弗拉克(x)参数及其标准不确定度通常受精修加权方案变化的影响不大，但如果有影响，那么数据或模型可能存在问题。精修后分析得到的弗拉克参数受权重选择的影响很大。从主要最小二乘法中使用的权重导出的权重，会导致弗拉克参数及其标准不确定度的精修后估计值与直接精修得到的非常相似。从观测结构振幅的方差导出的权重更合适，并且通常会产生与直接精修相似的精修后弗拉克参数，但标准不确定度总是更低。直接测定和精修后测定之间的显著差异强烈表明数据存在问题，而这些问题可能难以识别。提供了从28个结构测定中抽取的示例，展示了一系列不同的潜在问题。目前，考虑正态概率图斜率的精修后方法似乎是绝对结构最稳健的估计方法，应与直接精修值一起报告。
