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Dear all, Gerard Kleywegt, Bernhard Rupp and also John Helliwell explained to me that the unit of f' and friends is indeed meant to be electrons as in the elementary particle and not electrons as charge unit as in eV. Personally I find this very irritating and such things should be avoided - the formulae wouldn't change by using e (as in charge) as unit and adding a minus-sign. I should remember that a charge density map has negated signs compared to an electron density map. But I admit this is my personal view and might start a lengthy discussion about units as - if I remember correctly - we had on this board not long ago. It's just like my disliking that negative charge seems red for chemists and positive charge seems blue. Cheers, Tim On Fri, Feb 26, 2010 at 10:01:45AM +0100, Gerard DVD Kleywegt wrote: > Hi Tim, > > Maybe it's too early in the day for me, but why can't electrons be a > unit? You seem to be confusing physical (in-)divisibility of an entity > with the symbolic use of fractions of that entity in calculations. We can > speak of the average number of cows per acre of land without having to > cut up cows into small pieces (although I love a good steak as much as > the next person - and probably a lot more than that), or the average > number of people on a plane without having to remove some limbs of a > particular person to represent that number (although amputation of my > legs would make my journeys a lot more comfortable in terms of legroom). > > --dvd > > Disclaimer: this answer does not involve any (mention of) CCP4 software. > Mea culpa. > > > > On Fri, 26 Feb 2010, Tim Gruene wrote: > >> Dear all, >> >> I just stumbled across the question about what is the unit of f' and >> f''. The >> first couple of hits from ixquick.com claim it was e^-. Since e^- is >> not a unit >> but symbolises an elemtary particle (of which fractions are considered >> non-existent), I was wondering whether the unit of f, f', and f'' is >> actually e >> (a positive charge!) and the value of f^0 of Fe at its K-edge was >> actually 26e >> or -26e - see e.g. Table 1 in >> http://www.ccp4.ac.uk/courses/proceedings/1997/j_smith/main.html >> >> Cheers, Tim >> >> -- >> Tim Gruene >> Institut fuer anorganische Chemie >> Tammannstr. 4 >> D-37077 Goettingen >> >> GPG Key ID = A46BEE1A >> >> > > > Best wishes, > > --Gerard > > ****************************************************************** > Gerard J. Kleywegt > Dept. of Cell & Molecular Biology University of Uppsala > Biomedical Centre Box 596 > SE-751 24 Uppsala SWEDEN > > http://xray.bmc.uu.se/gerard/ mailto:[log in to unmask] > ****************************************************************** > The opinions in this message are fictional. Any similarity > to actual opinions, living or dead, is purely coincidental. > ****************************************************************** -- -- Tim Gruene Institut fuer anorganische Chemie Tammannstr. 4 D-37077 Goettingen GPG Key ID = A46BEE1A