Re: [sdpd] Best Cell?

[Armel Le Bail <armel...@fluo.univ-lemans.fr>]

>I wish to know what are criteria for 
>verifying the best cell suggested by 
>autoindexing. Especially, when there 
>are cells with V, 2V, 4V.

The principle is that you have to choose the proposition with the
highest symmetry explaining all your reflections.

It is always possible to index the reflections corresponding to a high
symmetry cell with a A, B, C, I, F or R Bravais lattice by a smaller 
symmetry P cell with smaller volume. Example, a rhombohedral
cell can always be described by a monoclinic C-centered cell with
2/3 volume ; a C-centered monoclinic cell is equivalent to a triclinic
cell with 1/2 volume, etc.

It is also always possible to index an hexagonal or trigonal cell
by an orthorhombic cell with a doubled volume.

Most indexing programs will recognize automatically such special
conditions. However, be very careful, having in mind the above
possibilities.

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.

Best,

Armel Le Bail
http://www.cristal.org/course/


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