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How to calculate sigmaa weighting map?

Sun May 3 11:03:37 EST 1998


I looked up in the old archive discussing sigmaa weighting map and 
noticed that Mr. Marcelo C. Sousa told that there were some errors in the
tutorial file $XPLOR/tutorial/xtal_maps/nfo-mfc_phicalc_map.inp of
Xplor3.851. the relative lines are:
      do (diff = combine($nn * fom * ampl(fobs) - $mm * dd * ampl(fcalc),
phase(fcalc)) )          ( acentric and sel=1 ) 
      do (diff = combine(fom * ampl(fobs), phase(fcalc)) ) ( centric and
sel=1 ) 

Mr. Marcelo C. Sousa suggested they should be corrected as:

do (deri1=combine(2 $nn fom ampl(fobs) - 2 $mm dd ampl(fcalc+fpart),
phase(fcalc+fpart))) ($low_res >= d >= $high_res and acentric and
ampl(fobs) > 0 ) 
do (deri1=combine($nn fom ampl(fobs) - $mm dd ampl(fcalc+fpart),
phase(fcalc+fpart))) ($low_res >= d >= $high_res and centric and
ampl(fobs) > 0 ) 

I also saw Mr. Eckhard Hofmann's article which analized this problem
and raised a question to the solution. But I did not see any answer to
his question. I would like to know which weighting scheme is correct.
Could any experts help me please?
When I use Xplor3.8's tutorial file to calculate Fo-Fc map, I got very
noisy map, even at 3 sigma level. But this file can give fairly good 
2Fo-Fc map. So I think its weighting scheme for centric reflections maybe 

Do you have any suggestion? think you very much.

  Xiaozhou Liu
  Biophysics and Physiology Department
  College of Life Science
  Peking University
  Beijing 100871, P.R.China
  E-mail: liuxz at iris.lsc.pku.edu.cn
          liuxz at pubms.pku.edu.cn
  Telephone: +86 10 62751864

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