[Wien] Freestanding Pb slab calculation

[Peter Blaha]

The structure is still wrong. You should "see" it in xcrysden.
The atoms must be at (0,0), (1/3,2/3) and (2/3,1/3) (A,B,C)
and not at (1/3,1/3),...

You cannot simply change the angle gamma from 60 to 120 without changing also
the positions.

I think it is pretty simple:
Create the structure with octave (same way as you did before).
Run sgroup and accept the structure.

It gives a nice hexagonal structure.



Am 08.09.2010 14:26, schrieb 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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>>
>
>
>
>
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-- 

                                       P.Blaha
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Peter BLAHA, Inst.f. Materials Chemistry, TU Vienna, A-1060 Vienna
Phone: +43-1-58801-15671             FAX: +43-1-58801-15698
Email: blaha at theochem.tuwien.ac.at    WWW: http://info.tuwien.ac.at/theochem/
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