... 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:[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
>
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