Flack H D, Bernardinelli G, Clemente D A, Linden A, Spek A L
Laboratoire de Cristallographie, University of Geneva, Switzerland.
Acta Crystallogr B. 2006 Oct;62(Pt 5):695-701. doi: 10.1107/S0108768106021884. Epub 2006 Sep 18.
The behaviour of the Flack parameter for centrosymmetric and pseudo-centrosymmetric crystal structures based on crystal structures published as being non-centrosymmetric is presented. It is confirmed for centrosymmetric structures that the value obtained for the Flack parameter is critically dependent on the Friedel coverage of the intensity data, approaching 0.5 for a coverage of 100% and sticking near the starting value for a coverage of 0%. For pseudo-centrosymmetric structures, even those very close to being centrosymmetric, it is found that it is often possible to obtain significant values of the Flack parameter. A theoretical basis for this surprising result is established. It has also been possible to establish an a priori estimate of the standard uncertainty of the Flack parameter based only on the chemical composition of the compound and the wavelength of the radiation. The paper concludes with preliminary presentations of bias in the Flack parameter and of inconsistent chemical and crystallographic data.
基于已发表的非中心对称晶体结构，给出了中心对称和伪中心对称晶体结构的弗拉克参数行为。对于中心对称结构，已证实弗拉克参数的值严重依赖于强度数据的弗里德尔覆盖度，覆盖度为100%时接近0.5，覆盖度为0%时接近起始值。对于伪中心对称结构，即使是非常接近中心对称的结构，也发现常常能够得到显著的弗拉克参数值。为此惊人结果建立了理论基础。还能够仅基于化合物的化学成分和辐射波长对弗拉克参数的标准不确定度进行先验估计。本文最后初步介绍了弗拉克参数的偏差以及不一致的化学和晶体学数据。
