> -----Original Message-----
> From: [log in to unmask] [mailto:[log in to unmask]]
> Sent: 12 January 2009 22:35
> To: Ian Tickle
> Cc: [log in to unmask]
> Subject: Re: [ccp4bb] structure (factor) amplitude
>
> Ian Tickle wrote:
>
> OK, limiting the vote to people whom I think we can assume know what
> vaguely they're talking about, i.e. Acta Cryst. / J. Appl. Cryst.
> authors, and using the IUCr search engine we get 553 hits for
> "structure
> amplitude" and 256 for "structure factor amplitude"
>
> But be warned that not all Acta Cryst. authors give the term
> "structure amplitude" the meaning that you think they do, i.e. a
> shortcut version for "structure factor amplitude" !
Actually it seems that "structure amplitude" in the |F| sense was
historically derived as a shortcut version of "magnitude of the
structure amplitude factor", not of "structure factor amplitude" which
is obviously derived directly from "structure factor". Similarly
"structure factor" was derived as a shortcut version of "structure
amplitude factor", but it has now unfortunately acquired at least 2
entirely different meanings as you point out.
The term "structure amplitude" clearly goes back a long way, it's much
older than "structure factor amplitude", and originally meant something
else entirely, as Gerard pointed out, so again we have ambiguous
terminology!
"Structure factor amplitude" (or maybe "modulus" or "magnitude" would
be better to avoid the historical ambiguities surrounding the term
"amplitude") has the clear advantage that it is relatively recent
terminology and hasn't had time yet to acquire ambiguous meanings, so I
would vote for it on that count alone!
Cheers
-- Ian
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