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 <[log in to unmask]>:
> <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
>
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