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SUMMARY:X-ray diffraction data reduction: space group determination\, scal
 ing and intensity statistics - Dr Phil Evans\, FRS\, LMB
DTSTART:20121010T091500Z
DTEND:20121010T101500Z
UID:TALK40326@talks.cam.ac.uk
CONTACT:23791
DESCRIPTION:Raw diffraction intensities from an integration program are no
 t usually on the same scale\, due to a number\nof physical factors affecti
 ng the experiment. The process of data scaling\, such as that done in the\
 nprogram AIMLESS or SCALA\, attempts to remove the systematic differences 
 from the intensities by modelling\nthe experiment\, putting them all on th
 e same scale\, or at least a self-consistent scale. Scaling relies on\nrep
 licate measurements related by crystal symmetry\, so it is important befor
 e scaling to determine the\npoint-group symmetry of the diffraction patter
 n\, and it is useful to choose a likely space group at this\nstage. The li
 kely point-group symmetry (the Laue group plus any lattice centring) is de
 termined by the\nprogram POINTLESS which compares reflections potentially 
 related by symmetry to see if they agree\, and\nranks the possible Laue gr
 oups by scoring their agreement. It is often also possible to determine th
 e\nspace group by analysis of axial systematic absences.\nScaling tries to
  make symmetry-related observations equal\, but analysis of the residual d
 ifferences after\nscaling provides a great deal of information about the q
 uality of the data\, both globally and locally. The\nintensities can be an
 alysed to set the real resolution of the dataset\, to detect bad regions (
 e.g. bad\nimages)\, to analyse radiation damage and to assess the overall 
 quality of the dataset. The signiï¬cance of\nany anomalous signal may be
  assessed by probability and correlation analysis. Analysis of distributio
 ns of\nthe averaged intensities can detect crystal pathologies such as twi
 nning and non-crystallographic\ntranslations\n \nThis lecture will present
  some examples of good and bad data and give some guidance on what to look
  out\nfor and on the decisions that need to be made.\n\n
LOCATION:Perham's seminar room
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