[QE-users] Symmetry Considerations

[Malte Sachs]

Dear Willem,

in case of a screw axis or a glide plane you couple a rotation or a 
mirrow with a fractional translation (e.g. 1/2 or 1/3 of a lattice vector).

Best,
Malte


Am 11.01.2019 um 09:03 schrieb Offermans Willem:
> Dear Paolo and Quantum Espresso friends,
>
> I’m sorry, but your explanation did not enlighten me.
>
> Your response: "it means that the symmetry operation consists in a 
> rotation (or a rotation + spatial inversion) plus a translation (not a 
> lattice one)” don’t ring a bell.
>
> What kind of translational symmetry are you talking about, if not the 
> lattice one?
>
> My system has S2-Symmetry. This means a rotation about 180°, followed 
> by reflection in a plane perpendicular to the rotation axis. It can be 
> shortened to an inversion centre. Combined with the identity 
> operation, this leads to 2 Sym.Ops, which have been detected by the 
> program. This is consistent with the message in the output. Since I’m 
> dealing with a slab, the system also has translational symmetry. I 
> don’t see why the inversion centre should be incommensurate with this 
> translational symmetry.
>
> What is the connection to the FFT grid?
>
>
>
>
> Met vriendelijke groeten,
> Mit freundlichen Grüßen,
> With kind regards,
>
>
> Willem Offermans
> Researcher Electrocatalysis SCT
> VITO NV | Boeretang 200 | 2400 Mol
> Phone:+32(0)14335263 Mobile:+32(0)492182073
>
> Willem.Offermans at Vito.be <mailto:Willem.Offermans at Vito.be>
>
>
>
>> On 10 Jan 2019, at 22:00, Paolo Giannozzi <p.giannozzi at gmail.com 
>> <mailto:p.giannozzi at gmail.com>> wrote:
>>
>> On Thu, Jan 10, 2019 at 4:30 PM Offermans Willem 
>> <willem.offermans at vito.be <mailto:willem.offermans at vito.be>> wrote:
>>
>>          2 Sym. Ops., with inversion, found ( 1 have fractional
>>     translation)
>>     [...] What does it mean?
>>
>>
>> it means that the symmetry operation consists in a rotation (or a 
>> rotation + spatial inversion) plus a translation (not a lattice one)
>>
>>     At some point in the sequence of calculations, the output
>>     contained the following message about symmetry:
>>
>>         No symmetry found
>>
>>     I was surprised about the lost of symmetry. Is it related to the
>>     fractional translation?
>>
>>
>> no, unless this happens in the last scf step, in which the FFT grid 
>> may change, in which case this is what may happen:
>>
>>   (note:  1 additional sym.ops. were found but ignored
>>            their fractional translations are incommensurate with FFT 
>> grid)
>> Paolo
>> -- 
>> Paolo Giannozzi, Dip. Scienze Matematiche Informatiche e Fisiche,
>> Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
>> Phone +39-0432-558216, fax +39-0432-558222
>>
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