2QVO.xds: Difference between revisions

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Revision as of 22:11, 14 March 2011

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→‎data reduction

Kay

Bureaucrats

2,652

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 The safest way to optimize the data reduction is to look at external quality indicators. Internal R-factors, and even the correlation coefficient of the anomalous signal are of comparatively little value. A readily available external quality indicator is CC All/CC Weak as obtained by [[ccp4com:SHELX_C/D/E|SHELXD]], and the percentage of successful trials.
-WFAC1 was already discussed above. Another candidate for optimization is MAXIMUM_ERROR_OF_SPOT_POSITION. By default this is 3.0 . In the case of these data, this default appears to be too small, because the STANDARD DEVIATION OF SPOT POSITION (PIXELS) (as reported by IDXREF, INTEGRATE and CORRECT after refinement) is quite high (1.5 and more). This prevents XDS from using all the reflections for geometry refinement. In general, it makes sense to use MAXIMUM_ERROR_OF_SPOT_POSITION= (at least 3 times the STANDARD DEVIATION OF SPOT POSITION (PIXELS))
+I tried a number of possibilities:
+* [[Optimization]] by "re-cycling" GXPARM.XDS to XPARM.XDS and re-running INTEGRATE, coupled with REFINE(INTEGRATE)= ! (empty list) and specifying BEAM_DIVERGENCE_E.S.D. and similar parameters as obtained from INTEGRATE.LP: this quite often helps to improve geometry a bit but had no clear effect here.
+* STRICT_ABSORPTION_CORRECTION=TRUE - this is useful if the chi^2 -values of the three scaling steps in CORRECT.LP are 1.5 and higher which is not the case here. Consequently this also had no clear effect.
+* increasing MAXIMUM_ERROR_OF_SPOT_POSITION from its default of 3 to ( 3 * STANDARD DEVIATION OF SPOT POSITION (PIXELS)) which would mean increasing to 5 here: no clear effect.
+* increasing WFAC1 : this was suggested by the number of misfits which is clearly higher than the usual 1 % of observations. WFAC1=1.5 has indeed a very positive effect on SHELXD: for dataset 1, the best CC All/Weak becomes 44.93 / 22.82 (dataset 2: 48.11 / 27.78), and the number of successful trials goes from about 60% to 91% (dataset 2: 94%).''' One should note that all internal quality indicators get worse when increasing WFAC1 - but the external ones got significant better!''' The number of misfits with WFAC1=1.5 dropped to 196 / 436 for datasets 1 and 2, respectively.
-I found that MAXIMUM_ERROR_OF_SPOT_POSITION=6.0 significantly improved the internal statistics (mostly the r-factors, but not so much the correlation coefficient of the anom signal), and improved CC All/CC Weak indicators (to more than 40). SHELXE then produces significantly better and more complete models. Try for yourself!
+We may thus conclude that in this case the rejection of misfits beyond the target value of 1% reduces data quality significantly. If no successful trials are made by SHELXD it may be worth to always try WFAC1=1.5 if the number of misfits is high.
-[[Optimization]] does improve things as much as it often does: recycling of GXPARM.XDS to use as XPARM.XDS, and thus imposing the lattice symmetry in the geometry refinement in INTEGRATE. These findings my correspond to the fact that in P1 the angles do not refine to 90.0xx or 89.9xx degrees. In other words, the metric symmetry is not as well fulfilled as it should be. This results in fairly large deviations from the ideal P42 positions; the refinement of cell parameters in P1 partly compensates for this. I have however no idea why this deviation from metric symmetry could occur.
 ===structure solution===