PROC-NAME: DIFMX01 Type_2

PURPOSE: To check that _refine_diff_density_max is within expected limits.

PROCEDURE:

DMAX = _refine_diff_density_max

DTEST = 0.1 * ZMAX

TEST

IF DMAX > DTEST*2 issue ALERT A

? ? ? ? ?"Alert A The maximum difference density is > 0.1*ZMAX*2.00"

> DTEST issue ALERT B

? ? ? ???"Alert B The maximum difference density is > 0.1*ZMAX*1.00"

> DTEST*.75 issue ALERT C

? ? ? ?? "Alert C The maximum difference density is > 0.1*ZMAX*0.75"

? ? ? ? The maximum residual electron density is larger than normally expected, even after making an allowance for the heaviest element in the structure. This is often an indication that (a) the absorption corrections are inadequate; (b) the overall quality of the data may be poor, leading to spurious peaks and holes of residual electron density; (c) there is twinning which has not been allowed for where overlap from the second twin domain (which may have been ignored in the data collection) causes errors in the intensities of some reflections; (d) the model is incorrect or incomplete in terms of incorrect element assignment, missing atoms or unmodelled or inadequately modelled disorder or solvent atoms.

? ? ? ? If you believe you have eliminated all potential causes of this alert, but the maximum residual electron density still remains larger than normally expected, you must specify the name of the nearest atom to this minimum and its distance from the minimum.

IF DMAX < 0.0 issue ALERT A

? ? ? ?? "Alert A The maximum difference density is < 0.0"

? ? ? ? The maximum residual electron density should have a positive value. Have you reversed the minimum and maximum entries?

(http://journals.iucr.org/services/cif/checking/DIFMX_01.html)

目的：檢查_refine_diff_density_max是否在預(yù)期范圍內(nèi)。

過程：

DMAX = _refine_diff_density_max

DTEST = 0.1 * ZMAX

檢查

若DMAX大于DTEST*2，則發(fā)布A級警告

? ? ? ?? "Alert A The maximum difference density is > 0.1*ZMAX*2.00"

若DMAX大于DTEST，則發(fā)布B級警告

? ? ? ?? "Alert B The maximum difference density is > 0.1*ZMAX*1.00"

若DMAX大于DTEST*.75，則發(fā)布C級警告

? ? ? ?? "Alert C The maximum difference density is > 0.1*ZMAX*0.75"

? ? ? ? 即使考慮到結(jié)構(gòu)中最重的元素，最小殘余電子密度峰也比正常預(yù)期值更負(fù)。這通常表明：(a)吸收校正不充分；(b)數(shù)據(jù)的整體質(zhì)量可能較差，導(dǎo)致出現(xiàn)虛假的殘余電子密度峰(鬼峰)和殘余電子密度空穴；(c)由于第二孿晶組分(在數(shù)據(jù)收集中可能已被忽略)的重疊引起某些衍射點(diǎn)的強(qiáng)度出現(xiàn)錯(cuò)誤而導(dǎo)致不被允許的孿晶；(d)由于元素指認(rèn)錯(cuò)誤、原子缺失或未建?；驘o序處理不充分或溶劑原子導(dǎo)致的模型不正確或不完整。

? ? ? ? 若你認(rèn)為你已經(jīng)消除了導(dǎo)致此警告的所有潛在原因，但最小殘余電子密度峰仍然比預(yù)期值更負(fù)，則必須給出最接近此最小殘余電子密度峰的原子的名稱及它們之間的距離。

若DMAX小于0.0，則發(fā)布A級警告

? ? ? ?? "Alert A The maximum difference density is < 0.0"

? ? ? ? 最小殘余電子密度應(yīng)為負(fù)值。檢查是否忘記負(fù)號(hào)或?qū)⒆钚『妥畲笾堤罘戳恕?/span>

例：DIFMX01_ALERT_2_B The maximum difference density is > 0.1*ZMAX*1.00

_refine_diff_density_max given = 0.982

Test value = 0.700

警告原因：最小殘余電子密度值超出預(yù)期范圍。

解決策略：可能的解決辦法：吸收矯正；數(shù)據(jù)不好，重新收集數(shù)據(jù)；模型錯(cuò)誤，包括歸屬錯(cuò)誤、原子丟失、無序或溶劑處理有誤。若無法更正，則指定最接近此最小殘余電子密度峰的原子名稱及它們之間的距離。

關(guān)于該警告的一個(gè)具體解釋例子參考下面鏈接中的公眾號(hào)文章： https://mp.weixin.qq.com/s/irYbkyXuk1kRluJryBZYeg