[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: [sdpd] Best Cell?
> Hope Robin Shirley agrees, he is probably writing a better answer
> than mine.
In fact I do pretty much agree, both with your reply and with
Lachlan's, (though don't hexagonal cells give alternative *V/2*
orthorhombic settings rather than 2V ones?).
> The standard Chekcell uses "parsimony of extra" reflections on all
> available cell and spacegroup combinations as a selection criteria
> of useful cells that could potentially be the "best cell".
Regarding this criterion, used in Chekcell and referred to by
Lachlan...
Something very similar was tried around 1970, by early zone-indexing
programs from the Delft group, for selecting zones according to what
they called "maximum coverage". When these programs evolved into
ITO, this was abandoned and a (somewhat over-optimistic) probability
estimate used instead. In its turn, this became supplanted in LZON
and FJZN6 by the Japanese PM criterion.
This "parsimony" or "coverage" criterion has its attractions, but
can also sometimes mislead, since it makes little use of the
quantitative goodness of fit between the observed and calculated
patterns.
Another methodological problem (shared by those versions of M20 that
allow the exclusion of "unindexed lines") is that the decision as to
whether or not an observed line is "indexed" by a theoretical
(calculated) one is essentially arbitrary, involving some threshold
or cut-off point in Q or 2Theta difference, beyond which, quite
abruptly, indexing is disallowed.
By contrast, criteria like PM make a much smoother transition and
handle unindexed lines more gracefully.
I hope to have time, next year perhaps, to explore the potential of
PM (or related criteria) for solution evaluation, and perhaps
implement them in further releases of Crysfire.
> A pity that so many experts prefer to stay at long distance (almost
> infinite ;-) from the Internet.
Not personally guilty, I hope.
I'm a moderately frequent contributor to these discussions (when I
feel I have have something relevant to say), and of course the free
non-profit release of Crysfire is published entirely on the Internet
via the CCP14 website.
Best wishes and a merry Xmas to all sdpd indexers
Robin Shirley
P.S. If you index protein powder patterns (or any cells with
volumes greater than c.5000 A**3), check out my new UNSCALE program
which restores the original scaling to rescaled Crysfire summary
files. This will be available shortly from the CCP14 website.
------------------------------------------------------------
To: sdpd...@egroups.com
From: Armel Le Bail <armel...@fluo.univ-lemans.fr>
Date: Fri, 15 Dec 2000 17:03:10 +0100
Reply-to: sdpd...@egroups.com
Subject: Re: [sdpd] Best Cell?
Hope Robin Shirley agrees, he is probably writing a better answer
than mine. A pity that so many experts prefer to stay at long distance
(almost infinite ;-) from the Internet.
-------------------------- eGroups Sponsor -------------------------~-~>
eLerts
It's Easy. It's Fun. Best of All, it's Free!
http://click.egroups.com/1/9699/1/_/40740/_/976911785/
---------------------------------------------------------------------_->