... and positive difference density should be red not green :-) Phil On 26 Feb 2010, at 09:22, Tim Gruene wrote:
> 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:[email protected] >> ****************************************************************** >> 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 >
