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小木虫论坛-学术科研互动平台 » 材料区 » 晶体 » 文献解析 » 【求助】编辑得回信的一个问题
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4220824

木虫 (正式写手)

早起的虫儿被鸟吃

[交流] 【求助】编辑得回信的一个问题

这是编辑的回信
Dear Professor Wang
I am reviewing your submission ，just now.
It seems that additional data have been collected to include a substantial proportion of the possible Friedel pairs. That allowed you to refine the structure to a reasonable data/parameter ratio of 1820/146  = "12.5" and also to use refinement of the Flack parameter "0.04(12)" to provide a credible determination of the absolute structure.
Would you please tell me "the number of Friedel pairs measured" to determine the absolute structure ?

With my best wishes,

因为我不是单晶专业的，有些术语看不懂，想问问，他最后一个问，是想让我做什么呢？
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1楼 2009-02-20 12:44:33
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James357

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4220824(金币+5,VIP+0):谢谢帮忙 2-20 18:25
你用于计算结构时候的 Friedel pairs值是多少。
你用Friedel pairs值计算了结果并用其它参数判断其合理性，可是你没有提供Friedel pairs是多少。

[ Last edited by James357 on 2009-2-20 at 14:36 ]
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2楼2009-02-20 14:32:07
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xi2004

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这个CCDC自己有个很具体的说明,如何来算 Friedel pairs ,
我记得我在某个贴子里贴过的.
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xi2004-emuch@163.com
3楼2009-02-20 15:04:49
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xi2004

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lingzi326(金币+1,VIP+0):谢谢热心回复 2-20 20:40
http://journals.iucr.org/services/cif/checking/STRVA_01.html

PROC-NAME: STRVAL_01 (FL)

PURPOSE: To check that _refine_ls_abs_structure_flack is within expected limits.

PROCEDURE:
FLACK = _refine_ls_abs_structure_flack
SFLACK = su of _refine_ls_abs_structure_flack

TEST
IF FLACK > 0.7 issue ALERT C
   "Alert C Chirality of atom sites is inverted?"
The correct absolute structure has been defined by the atomic coordinates if _refine_ls_abs_structure_flack is close to 0.0 (and the s.u. is sufficiently small). If _refine_ls_abs_structure_flack is close to 1.0, the incorrect enantiomer is being modelled and the atomic coordinates should be inverted and refined again.
In cases of intermediate values of _refine_ls_abs_structure_flack, and you believe that a merohedral twin or a partial mix of enantiomers is present, choose the configuration that gives the lowest value for _refine_ls_abs_structure_flack.

Note that if the s.u. of the Flack parameter is large, e.g. greater than or equal to 0.3, one cannot confidently derive the absolute structure from the data, because, within the 3sigma confidence limits, the full range of possible values of the Flack parameter are plausible. In such cases, even if the Flack parameter itself is close to 0.0, no conclusions about the absolute structure are justified. This will usually be the case if the compound is a weak anomalous scatterer (i.e. no atom heavier than Si is present). If a heavy atom is present, but the s.u. is still large, you should ensure that a significant fraction (ideally all) of the Friedel opposites of the symmetry unique reflections have been included in the data set.

If the absolute structure parameter is meaningless because the compound is a weak anomalous scatterer, it is best to remove the absolute structure parameter from the CIF.

If the absolute structure parameter is meaningful, please also provide appropriate reference details under _refine_ls_abs_structure_details. For example:

  _refine_ls_abs_structure_details 'Flack (1983), XXXX Friedel pairs'

and replace the XXXX with the actual number of Friedel pairs used in the refinement. [An easy way to determine the number of Friedel pairs is to look at the difference between the number of unique reflections used in SHELXL when a MERG2 and MERG 3 instruction is used (MERG 3 forces Friedel pairs to be merged before use).]

ELSE IF FLACK > 0.3 AND FLACK < 0.7 issue ALERT C
   "Alert C Flack test results are ambiguous."
The correct absolute structure has been defined by the atomic coordinates if _refine_ls_abs_structure_flack is close to 0.0 (and the s.u. is sufficiently small).
In cases of intermediate values of _refine_ls_abs_structure_flack, a merohedral twin or a partial mix of enantiomers may be present and this fact should be discussed in the manuscript. However, intermediate values might also be obtained when the absolute structure parameter is essentially meaningless because the compound is a weak anomalous scatterer.

Note that if the s.u. of the Flack parameter is large, e.g. > 0.3, one cannot confidently derive the absolute structure from the data, because, within the 3sigma confidence limits, the full range of possible values of the Flack parameter are plausible. In such cases, even if the Flack parameter itself is close to 0.0, no conclusions about the absolute structure are justified. This will usually be the case if the compound is a weak anomalous scatterer (i.e. no atom heavier than Si is present). If a heavy atom is present, but the s.u. is still large, you should ensure that a significant fraction (ideally all) of the Friedel opposites of the symmetry unique reflections have been included in the data set.

If the absolute structure parameter is meaningless because the compound is a weak anomalous scatterer, it is best to remove the absolute structure parameter from the CIF.

If the absolute structure parameter is meaningful, please also provide appropriate reference details under _refine_ls_abs_structure_details. For example:

  _refine_ls_abs_structure_details 'Flack (1983), XXXX Friedel pairs'

and replace the XXXX with the actual number of Friedel pairs used in the refinement. [An easy way to determine the number of Friedel pairs is to look at the difference between the number of unique reflections used in SHELXL when a MERG2 and MERG 3 instruction is used (MERG 3 forces Friedel pairs to be merged before use).]

ELSE IF FLACK < -0.2 issue ALERT C
   "Alert C Flack parameter is too small."
The value of _refine_ls_abs_structure_flack is well outside the expected range of 0.0-1.0. This may mean that there is some systematic error in the model or data, or that the absolute structure parameter is meaningless because the compound is a weak anomalous scatterer. In the latter case, the s.u. of the Flack parameter is also usually large and it is best to remove the absolute structure parameter from the CIF. If a heavy atom is present in the compound, you should ensure that a significant fraction (ideally all) of the Friedel opposites of the symmetry unique reflections have been included in the data set. If _refine_ls_abs_structure_flack is still out of range, the quality of the data and/or the correctness of the model could be checked carefully.  
ELSE IF SFLACK > 0.5 issue ALERT C
   "Alert C Flack test results are meaningless."
If the s.u. of the Flack parameter is large, e.g. > 0.3, one cannot confidently derive the absolute structure from the data, because, within the 3sigma confidence limits, the full range of possible values of the Flack parameter are plausible. In such cases, even if the Flack parameter itself is close to 0.0, no conclusions about the absolute structure are justified. This will usually be the case if the compound is a weak anomalous scatterer (i.e. no atom heavier than Si is present). If a heavy atom is present, but the s.u. is still large, you should ensure that a significant fraction (ideally all) of the Friedel opposites of the symmetry unique reflections have been included in the data set.
If the absolute structure parameter is meaningless because the compound is a weak anomalous scatterer, it is best to remove the absolute structure parameter from the CIF.



--------------------------------------------------------------------------------
Full list of validation algorithms
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xi2004-emuch@163.com
4楼2009-02-20 15:19:10
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xi2004

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4220824(金币+5,VIP+0):呵呵，给的信息很全，谢谢了 2-20 18:26
[An easy way to determine the number of Friedel pairs is to look at the difference between the number of unique reflections used in SHELXL when a MERG2 and MERG 3 instruction is used (MERG 3 forces Friedel pairs to be merged before use).]

就是这句话.
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xi2004-emuch@163.com
5楼2009-02-20 15:19:43
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