Strange Fo-Fc map

Eckhard Hofmann eckhard.hofmann at uni-konstanz.de
Mon Dec 15 08:24:36 EST 1997

I just picked up the thread about the map input script error. But I'am
not clear about the error in the script and the proposed cure ...
Maybe someone could enlighten me?
As far as I got it there was supposedly a mistake in the proper
treatment of centric and acentric reflections.
So the cure were the following lines:

acentric:  deri1=combine(2 \$nn fom ampl(fobs) - 2 \$mm dd
ampl(fcalc+fpart)
centric:   deri1=combine(\$nn fom ampl(fobs) - \$mm dd ampl(fcalc+fpart)

I assume the first line read as follows in the original script (which I
don't have here):
acentric: deri1=combine(\$nn fom ampl(fobs) - \$mm dd ampl(fcalc+fpart)

So the difference would be to apply a factor of two to the acentric
reflections which leads to a downweighting of centric reflections by the
same amount. Still the centrics are treated with the proper \$nn and \$mm.

If I look at the inpfile
\$XPLOR/tutorial/xtal_maps/nfo-mfc_phicalc_map.inp
I find the following :

acentric: diff = combine(\$nn * fom * ampl(fobs) - \$mm * dd *
ampl(fcalc), phase(fcalc))
centric: diff = combine(fom * ampl(fobs), phase(fcalc))

Here we take the acentrics with the true \$nn and \$mm, but we fix the
centrics to a weighting of 1. This results in no downweighting of
centrics in the case of an fo-fc map and variable degrees of
downweighting in other cases.
Did I get this right? If so, which manner is the 'correct' (whatever
that is ;-)) one?

Thanks for any hints...
Eckhard

--
Eckhard Hofmann           <eckhard.hofmann at uni-konstanz.de>
Fakultaet fuer Biologie
Universitaet Konstanz
Box M656
D-78457 Konstanz, Germany
Phone: voice: +49 7531 88-2211   fax: +49 7531 88-3183