Re: cheshire cell in AmoRe

[Ian J Tickle <i.tickle@mail.cryst.bbk.ac.uk>]

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On Tue, 11 May 1999, R. Ravishankar wrote:

>   It is mentioned in the AMoRe manual that
> The asymmetric unit that needs to be searched for the translation function
> (1-body) is the Cheshire cell while for n-body translation it is the whole
> cell.
>  Can someone enlighten me as to what constitutes the cheshire cell -Please

Jorge Navaza had a very comprehensive list showing all the Cheshire groups,
but I don't know if it's available somewhere on the Web (Jorge?).

Basically the Cheshire group is the space group of the crystal when its
material contents are removed leaving only the symmetry elements, like the
smile that was left when the Cheshire Cat disappeared.  (In case you're not
aware of the connection between Alice in Wonderland and CCP4, Charles
Lutwidge Dodgson, alias Lewis Carroll, was born in Daresbury which is located
in the English county of Cheshire).

The higher the space group symmetry, the lower is the symmetry of the
Cheshire group, e.g. for space group P1 the Cheshire group consists of a
single point (i.e. all points in the space are equivalent), which means that
in P1 the translation function for 1 mol./a.u. is already solved before you
start!  Conversely, for primitive cubic space groups, the Cheshire cell has
much lower symmetry, which means you have to search half the unit cell, even
with 1 mol./a.u.

In between these extremes there are all the other space groups.  For further
details look in my documentation for the TFFC program (you might even try
using the program!).

-- Ian
_______________________________________________________________________________

 Ian J. Tickle                           | Phone:  (+44) 171 631 6854
 Department of Crystallography           | Fax:    (+44) 171 631 6803
 Birkbeck College (University of London) | Email:  i.tickle@bbk.ac.uk
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