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## CRYSTALS Single Crystal Suite

### Using DIFABS in Crystals as a diagnostic test

The CCP14 Homepage is at http://www.ccp14.ac.uk

[Tutorials page] | [Crystals Tutorial page]

### Why bother using DIFABS Analysis?

"You can use DIFABS in test mode at any stage, and it quite simple tells you if there is a systematic trend to the residual as a function of orientation. If the plot is fairly monochrome, then the residuals are not peaking up. If there are bright patches, it means that there is a systematic trend to the residual. Either FO or Fc is systematically wrong. Possible reasons are:

• Absorption
• crystal damage (bending, shearing)
• anisotropic extinction
• mis-centering
• very anisotropic 'spot' shape (selective truncation)
• Application of unsuitable/ill-measured psi correction
(Use of DIFABS in 'test' mode to un-corrected and psi-corrected data can help display the effect of the correction more clearly than just looking at Rint)
• We have no evidence that missing atoms (eg solvent) lead to Difabs-like effect"

 To use DIFABS for diagnosis of your structure, from the main menu use Analysis, Difabs analysis

 Output from an inorganic Cesium Titanium Silicate after the final stages of refinement ```R-value= 3.906 Rw= 7.074 (Sum Fo)/(Sum Fc)= 1.00 Minimisation function= 1959. There are 0 singularities Sum of the squares of the ratio (Shift/e.s.d.) = 0.0000 The rms (shift/esd) = 0.000464 The rms (shift/esd) is less than the target ( 0.300000 ). Futher cycles abandoned Largest shift/esd (= 0.002275), for Parameter 101 O 12 U[23] Statistics for 112 least squares parameters, with 1762 degrees of freedom X-rays Restraints All Number of observations 1874 0 1874 Hamilton weighted R-value 7.074 0.000 7.074 ! 1360 #SCRIPT XDIFABS This SCRIPT runs DISABS in test mode. Use the SCRIPT DIFABS to apply the correction. DIFABS Version 9.01 Applied to Fo or Fc There is no orientation matrix. DIFABS correction will not be optimal 0 Processing reflections 100% DIFABS correction applied to Fo 0 Processing reflections 100% For the 1764 reflections the relative residual was 0.0343 and the relative rms residual was 0.0363 1874 Reflections corrected. Average correction = 0.998 (On Fo) Minimum correction = 0.958 Maximum correction = 1.088 ```

[Tutorials page] | [Crystals Tutorial page]

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If you have any queries or comments, please feel free to contact the CCP14