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# How to calculate sigmaa weighting map?

liuxz at PUBMS.PKU.EDU.CN liuxz at PUBMS.PKU.EDU.CN
Sun May 3 11:03:37 EST 1998

```Hi,

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
wrong.

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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