[QE-users] Symmetry Considerations

[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>

[cid:982BA063-B96A-4A1B-89AB-5A01CA9FC70D at vito.local]

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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