[Pw_forum] Different number of symmetry operations

[Paolo Giannozzi]

On Thu, 2013-04-18 at 15:30 -0400, David Strubbe wrote:

> Why are symmetries with fractional translations disabled
> in supercells?

because there is no simple way to ensure that a group of
point symmetries: rotations + fractional translations, is 
a group in the mathematical sense of the term. In a true 
cell, with no additional underlying translational symmetry, 
fractional translation are well-defined, while in a supercell 
there are several possible choices, differing for a lattice 
translation of the underlying cell. Years ago I didn't find 
a simple way to ensure that the choice of fractional 
translation was consistent with the properties of a symmetry 
group (if it isn't, the symmetrization procedure breaks down).
Given the limited interest of this specific case, I haven't 
inquired any further.

P.
-- 
Paolo Giannozzi, Dept. Chemistry&Physics&Environment, 
Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
Phone +39-0432-558216, fax +39-0432-558222