My two cents worth. If you look at, for instance,
https://en.wikipedia.org/wiki/List_of_space_groups, Cm #8 is a monoclinic
spacegroup which (in the B11m setting) has beta .ne. 90. It is "correct" to
have gamma .ne. 90 for the standard convention. However, this is
inconvenient and it would be nice if sgroup made an exception for Wien.

On Wed, Oct 12, 2016 at 10:14 AM, Peter Blaha <pbl...@theochem.tuwien.ac.at>
wrote:

> ===================================================================
>
> The structure that you have attached does NOT have Cmm2 as space group
> symmetry, but only Cm, which is the one that sgroup correctly finds. The
> mirror plane of the Cm group is perpendicular to the original a axis, and
> in the standard description of Cm it should be perpendicular to the b axis.
> That is the reason why it interchanges the axes, to make the old a axis
> become
> the new b one. It is however true that it could have however kept the
> original
> unit cell metrics, with the axes interchanged. The program indeed does not
> seem to have any particular criterion to optimize the choice of the unit
> cell within the standard description of the space group, and as Peter Blaha
> says there are many possible choices for monoclinic space groups. The
> program just chooses one, the reason being that your structure is
> monoclinic
> with Cm symmetry and not Cmm2.
>
> J. Manuel Perez-Mato
> Fac. Ciencia y Tecnologia,
> Universidad del Pais Vasco, UPV
> 48080 BILBAO,
> Spain
> ***************************************************
>
>
> On 10/12/2016 04:50 AM, delamora wrote:
> > I want to see how Cu interacts with the TiO2 surface, so I construct
> > with 'supercell' an enlarged cell, with
> > SG 35 (C m m 2)
> > then I attach Cu and O on the surface in a way that I break the symmetry
> > in the b direction
> > The cell is orthorhombic
> >
> > CXY LATTICE,NONEQUIV.ATOMS: 21
> > MODE OF CALC=RELA unit=bohr
> >  11.187184 23.838706 23.838290 90.0 90.0 90.0
> >
> > if I run initialize calculation and then sgroup the new cell changes to
> >
> > CXZ LATTICE,NONEQUIV.ATOMS: 28 8 Cm
> > MODE OF CALC=RELA unit=bohr
> >  23.838706 33.712727 11.187184 90.0 90.0 135.000500
> >
> > which is correct, but it is no longer orthogonal which is awkward, one
> > angle changes from
> > 90 to 135.0005
> > also the a, b, c => c, a, b
> >
> > Also I cannot see the structure with XCrySDen, unless I change the
> > 135.0005 angle by little.
> >
> > With the new structure and the old one I managed to make a new
> > TiO2.struct with orthorhombic and can be seen with XCrySDen
> >
> > It would be nice that sgroup would generate a structure with the
> > orthorhombic symmetry
> >
> >               Saludos
> >
> >                       Pablo de la Mora
> >
> >
> >
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> --
>
>                                        P.Blaha
> --------------------------------------------------------------------------
> Peter BLAHA, Inst.f. Materials Chemistry, TU Vienna, A-1060 Vienna
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-- 
Professor Laurence Marks
"Research is to see what everybody else has seen, and to think what nobody
else has thought", Albert Szent-Gyorgi
www.numis.northwestern.edu ; Corrosion in 4D: MURI4D.numis.northwestern.edu
Partner of the CFW 100% program for gender equity, www.cfw.org/100-percent
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