[Wien] Change of structure symmetry

[Peter Blaha]

It is not always mandatory to take the changes suggested by sgroup into 
account. (This is the reason why it is not made automatically). For low 
symmetry structures (monoclinic) there are many possible settings and 
even the construction of the unit cell is not unique, but arbitrary 
equivalent choices exists.

The following "rules" exist: Run

x nn   #  and accept all changes to get the proper multiplicity of the 
atoms.
x sgroup   #  and check if sgroup has reduced the unit cell (or changed 
the multiplicity). Only in this case you have to accept the changes by 
sgroup. ("Reduced" means: the total number of atoms in the cell got 
smaller). If sgroup just changed the lattice type or a,b,c,angles; there 
is no need to accept the scgroup changes.
To be sure that you can neglect the changes, just  remember the number 
of symmetry operations which sgroup has found.

x symmetry  # compare the number of symmetry operations from symmetry 
and sgroup. If this is the same (of course, the individual operations 
may be different) and symmetry also found identical site symmetry of all 
positions, you are save and can simply neglect sgroup.


Am 12.10.2016 um 04:50 schrieb delamora:
> 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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-- 
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Peter BLAHA, Inst.f. Materials Chemistry, TU Vienna, A-1060 Vienna
Phone: +43-1-58801-165300             FAX: +43-1-58801-165982
Email: blaha at theochem.tuwien.ac.at    WIEN2k: http://www.wien2k.at
WWW:   http://www.imc.tuwien.ac.at/staff/tc_group_e.php
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