[Wien] Freestanding Pb slab calculation

[Bartosz Slomski]

After executing x sgroup with my initial Pbslab_34_34.struct I get the 
warning: !!! Bravais lattice has changed.
sgroup found: 12 (C 2/m) [unique axis c] cell choice 2.


When I accept the structure of x sgroup, then the resulting structure 
(Pb495_after_symmetry.struct) looks completely different from the 
original one (Pbslab_34_34.struct)

But, when I take the Au example and go through the initialization then 
the message out of sgroup is just. symmetry group found sgroup 164 (P -3m 1)

So there must be something wrong in my initial structure that I dont 
understand.

When I create the slab with octave and load into structgen then 
StructGen says about the lattice: nie_mam pojecia, which means, "dont 
know the spacegroup", then I edit the struct.file to lattice H and 
change gamma from 60° to 120°.
But then I get the error: incorrect space group symbol.

Sorry for boreing you with all these questions and problems...perhabs 
somebody can provide me the commands to produce a 111 surface?

With best regards,

Bartosz



Peter Blaha wrote:
> The procedure is ok. But you should accept the changes of sgroup !
> It will move the origin so that you have inversion symmetry.
>
> Am 08.09.2010 12:15, schrieb Bartosz Slomski:
>> Dear Mr Blaha,
>>
>> indeed my structure does not represent a surface along [111].
>> I made a mistake using structgen/octave. Instead of using 
>> makeconventional one should use "makeprimitive"
>> Afterwards one has to change in StructGen the gamma angle to 120° and 
>> select lattice type H.
>>
>> The final structure (Pb_slab_34_34.struct) has the stacking of A-B-C 
>> but still differs slightly from the Au example.
>> In the Au example there is vacuum between the 12th layer and the 13th 
>> layer, or are the layers 13..24 just the repeated unit cell of layer 
>> 1..12?
>>
>> During initialization of the Pb_slab I get the warning: you must move 
>> the origin.
>>
>> Perhabs the procedure to generate a surface along [111] is still wrong.
>>
>> Thanks and best regards.
>>
>>
>> Peter Blaha wrote:
>>> Have you looked how the generated structure looks like ? (xcrysden)
>>>
>>> It is not correct.
>>>
>>> I'm not the expert with structgen and thus cannot tell you, where 
>>> the error is.
>>>
>>> In any case, an fcc-111 surface is very easy (although one cannot use
>>> directly the supercell program).
>>>
>>> Have a look into example_struct_files/Gold_23l.struct
>>> This is an fcc 111 structure. All you need to do is to adjust the
>>> lattice parameters and eventually delete some of the atoms (if you 
>>> want just
>>> 6 layers instead of 23).
>>>
>>> Remember the stacking: A-B-C .... !
>>>
>>> PS. In general it is NOT a good idea to neglect 
>>> warnings/errors/suggestions
>>> from nn or sgroup, unless "you know why you are doing it".
>>>
>>> A 6-layer slab has inversion symmetry and only 6 atoms/cell !
>>> It runs in a couple of minutes, not 19 h.
>>>
>>>
>>
>>
>>
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>


-- 
Bartosz Slomski
------------------------------------------
Swiss Light Source, Paul Scherrer Institut
WSLA/205
CH-5232 Villigen PSI, Switzerland
Tel: +41 (0)56 310 5694
------------------------------------------
Physik-Institut, Universität Zürich
Winterthurerstr. 190
CH-8057 Zurich, Switzerland
Tel: +41 (0)44 635 5824

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