How many apples do you have? 1, 2, 3... this is unitless.
If using apple as currency, the answer will be that "apple" is unit.
Unit is a concept for information exchange. If f stands alone,
no one cares the unit.
Electron is the currency atoms used to communicate with X-ray,
its unit is ELECTRON.
Lijun
On Feb 26, 2010, at 2:04 PM, marc.schi...@epfl.ch wrote:
I fully agree with Ian and would again point to the authoritative
documentation :
http://www.bipm.org/en/si/derived_units/2-2-3.html
The quantities f^0, f' and f" are unitless, i.e. simply numbers (or
rather: their unit is the number one, which is usually omitted).
The unit of the electron density is really just 1/Å^3. To see this,
consider that the electron density is defined to be
\rho = (Number of electrons)/volume
The numerator is simply a count, and thus unitless (or rather: its
unit is the number one).
In practice, we like to a remind ourselves that these values refer to
electrons and therefore like to think of e/Å^3 as the unit of electron
density, but this is somewhat incoherent, if not incorrect. The fact
that we are dealing with electrons (as opposed to apples) is contained
in the definition of the quantity "electron density". It does not need
to be explicitly specified in the unit.
Marc
Quoting Bernhard Rupp <b...@ruppweb.org>:
<NOTATION>
Notation
========
f0: atomic scattering factor for normal scattering, defined as the
ratio
of scattered amplitude to that for a free electron.
</NOTATION>
----------------------------------------------------------------------
Hmmm...where does the 'electron' in electron density then come from
after
integration/summation over the structure factors?
----------------------------------------------------------------------
BR
Lijun Liu
Cardiovascular Research Institute
University of California, San Francisco
1700 4th Street, Box 2532
San Francisco, CA 94158
Phone: (415)514-2836
