DIFMN02

PROC-NAME: DIFMN02 Type_2

PURPOSE: 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.

IF DMIN > 0.0 issue ALERT A
"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?

Full list of validation algorithms