CheckCIF-PLAT021-030
PLAT021 Type_4 Check Expected number of Reflections (Max = 1 Centro, 2 - non-centro)
The expected number of reflections corresponds to that in the asymmetric unit of the Laue group. Expected ratio: less-or-equal 1 for centro symmetric structures and less than 2 for non-centrosymmetric structures. Reasons to exceed those numbers can be:
1 - Systematic extinctions not omitted from the observed data count.
2 - Refinement with redundant (i.e. not merged/unique) data set.
3 - SHELXL HKLF 5 Refinement.
(http://journals.iucr.org/services/cif/checking/PLAT021.html)
警告原因：针对富余的独立衍射点的测试。实际独立衍射点数在cif文件中_reflns_number_total项给出；期望独立衍射点数相当于Laue群中不对称单元的衍射点的个数，这个数值通过晶胞参数、空间群、theta max算出。对于中心对称空间群，二者的比应小于等于1；对于非中心对称空间群，二者的比应小于2。远超过上述数值会有Alert，产生的原因有：
1. 系统消光的衍射点未被扣除。
2. 使用未被合并的数据进行精修。
3. 使用HKLF 5格式的数据进行精修。
解决策略：一般不会出现。如果数据未合并需要合并数据。
PLAT022 Type_3 Check Expected number of Reflections (completeness)
Test for data completeness. The ratio of the reported number of unique reflections and expected number of reflections for the resolution given is reported. The ratio can be low due to a missing cusp of data when collected with a 2D-detector. Alternatively, the wrong asymmetric part of reciprocal space was collected on a serial detector system.
(http://journals.iucr.org/services/cif/checking/PLAT022.html)
警告原因：该项为针对缺少衍射点的测试。当实际独立衍射点远小于期望值时，会给出Alert。产生的原因有使用面探检查器收集数据缺失或者倒易空间收集错误。
解决策略：重新还原数据或者重新收集数据。
PLAT023 Type_3 Check Theta-Max
Check resolution of the data set. Alert is issued when sin(theta)/lambda < 0.6
(http://journals.iucr.org/services/cif/checking/PLAT023.html)
警告原因：检查数据分辨率。当sin(theta)/lambda < 0.6时表明数据分辨率低。
解决策略：重新还原数据或者重新收集数据。
PLAT024 Type_4 Check for required Friedel pair averaging Z<=Si (Obsolete 1/1/2011)
(No longer imposed)
(http://journals.iucr.org/services/cif/checking/PLAT024.html)
Averaging of Friedel pairs is advised when indicated by a large s.u. on the Flack parameter. Large s.u. values indicate that the anomalous scattering power is too small in combination with the data quality at hand to merit refinement with a non-averaged data set. This will generally be the case with MoKa data sets for structures containing atoms not heavier than Si. The value of the Flack parameter will be largely meaningless anyway for large s.u. values. Use MERG 3 or MERG 4 in case of refinement with the SHELXL97 program. Non-compliance with the above for valid scientific reasons should be discussed in detail in the experimental section of the paper.
例：PLAT024_ALERT_4_B Merging of Friedel Pairs is STRONGLY Indicated
警告原因：当Flack系数的误差较大时，表明反常散射能力弱。一般与体系中没有重原子有关。可以将Friedel对合并进行精修。
解决策略：一般不会出现。使用MERG 3或者MERG 4合并Friedel对重新精修即可。
PLAT025 Type_1 Check for Hmin..Lmax
Check reported h,k,l - range with calculated range based on reported theta-max.
(http://journals.iucr.org/services/cif/checking/PLAT025.html)
警告原因：报道的hkl范围和计算得到的hkl范围不一致。
解决策略：一般不会出现。精修重新生成CIF文件即可。
PLAT026 Type_3 Check for a weak data set
Check whether a sufficient fraction of the unique data set is indeed above the 2 * sigma(I) level.
All reflection data are included in this test. Check whether this low ratio is caused by weak data beyond sin(theta)/lambda > 0.5 (i.e. low resolution).
(http://journals.iucr.org/services/cif/checking/PLAT026.html)
例：PLAT026_ALERT_3_C Ratio Observed / Unique Reflections too Low ....47 Perc.
警告原因：独立衍射点中可观测独立衍射点比例低。
解决策略：可以尝试用SHEL 999 0.84命令或OMIT -2 50命令截去一部分高角度点，如果不行就只能重新收数据了。
PLAT027 Type_3 Check _diffrn_reflns_theta_full
Ideally (and a requirement for publication in Acta Crystallographica), the dataset should be essentially complete, as defined by -diffrn-measured-fraction-theta-full (close to 1.0), up to sin(theta)/lambda = 0.6 (i.e. 25.24 degrees MoKa).
The three major causes of incomplete data sets are:
1 - A missing cusp of data due to data collection by rotation around the spindle axis only (standard on some image-plate systems). Cure: collect an additional data set after remounting the crystal.
2 - The DENZO image processing package has problems with certain strong reflections. They are often excluded from the data set. Cure: Add an additional scan at lower power setting in order to include strong low order reflections.
3 - Incomplete scans.
(http://journals.iucr.org/services/cif/checking/PLAT027.html)
警告原因：当数据完整度接近1，theta角应满足sin(theta)/lambda大于等于0.6时，小于0.6时会给出Alert。数据完整度低的原因主要有：
数据收集策略时探测器只围绕主轴线转动，可以重新把晶体放置在载晶台上补充数据。
数据处理时强点由于无法处理被排除，可以减少曝光时间减弱衍射强度。
扫描不完整。
解决策略：可以尝试用SHEL 999 0.84命令或OMIT -2 50命令截去一部分高角度点，如果不行需调整曝光时间重新收集数据。
PLAT028 Type_3 Check the reported _diffrn_measured_fraction_theta_max
Ideally, the reported '_diffrn_measured_fraction_theta_max' value, corresponding to theta-max, should be close to 1.0.
(http://journals.iucr.org/services/cif/checking/PLAT028.html)
警告原因：theta-max角度下数据点部分缺失。
解决策略：使用SHEL或OMIT命令适当截去少量高角度点即可。
PLAT029 Type_3 Check the reported _diffrn_measured_fraction_theta_full
Ideally (and a requirement for publication in Acta Crystallographica), this fraction should be close to 1.0 for theta-full greater or equal to sin(theta/lambda) = 0.6 (i.e. 25.24 degrees for MoKa and 67.7 degrees for CuKa radiation).
The three major causes of incomplete data sets are:
1 - A missing cusp of data due to data collection by rotation around the spindle axis only (standard on some image-plate systems). Cure: collect an additional data set after remounting the crystal.
2 - The DENZO image processing package has problems with certain strong reflections. They are often excluded from the data set. Cure: Add an additional scan at lower power setting in order to include strong low order reflections.
3 - Incomplete scans, possibly based on erroneously assumed higher than actual symmetry.
Note: The default value of _diffrn_measured_fraction_theta_full that is automatically calculated and inserted in the CIF by SHELXL-97 might generate A-level ALERTS when significant numbers of reflections are missing at higher theta values. In order to avoid such an ALERT, substitute the values calculated with the SHELXL instruction 'ACTA 50' for _diffrn_reflns_theta_full and _diffrn_measured_fraction_theta_full respectively. For Mo-radiation, corresponding values of 25 degees (or higher) and 0.99 (or higher) are expected. (See SHELXL manual). PLATON may be used to analyse the case at hand (by invoking either the 'FCF-VALIDATION' mode or the 'ASYM-VIEW' mode).
(http://journals.iucr.org/services/cif/checking/PLAT029.html)
例：PLAT029_ALERT_3_A _diffrn_measured_fraction_theta_full Low ....... 0.93
警告原因：当theta-full满足sin(theta/lambda)大于等于0.6时，数据完整度低。原因同PLAT027相同。
解决策略：可以尝试用shel 999 0.84或omit 0 50或omit -3 50切去一部分高角度点，如果不行需调整曝光时间重新收集数据。
PLAT030 Type_1 Check _diffrn_reflns_number >= reflns_number_total
The number of measured reflections should be equal or greater than the number of unique reflections.
(http://journals.iucr.org/services/cif/checking/PLAT030.html)
警告原因：收集衍射点数应大于等于独立衍射点数。
解决策略：一般不会出现，重新精修生成CIF文件即可。
