menucheckCIF procedure
PROC-NAME: DIFMN02 Type_2PURPOSE: To check that _refine_diff_density_min is within expected limits.
PROCEDURE:
DMIN = _refine_diff_density_min
DTEST = 0.1 * ZMAX
TEST
IF DMIN < -DTEST*2 issue ALERT A
"Alert A The minimum difference density is < -0.1*ZMAX*2.00"
< -DTEST issue ALERT B
"Alert B The minimum difference density is < -0.1*ZMAX*1.00"
< -DTEST*.75 issue ALERT C
"Alert C The minimum difference density is < -0.1*ZMAX*0.75"
|
The minimum residual electron density is more negative
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 minimum residual electron density still remains more negative than normally expected, you must specify the name of the nearest atom to this minimum and its distance from the minimum. |
"Alert A The minimum difference density is > 0.0"
| The minimum residual electron density should have a negative value. Have you forgotten the sign or reversed the minimum and maximum entries? |
