Thanks Vahid! This is actually a nice hint. I didn't relax the lattice with QE since I got the relaxed structure from someone else (collaboration) and minor differences between codes had - in my experience (3D solids) - never such a big influence on the phonons modes. But I will have a look at the stresses...
Anyways, meanwhile the phonon dispersion was also calculated using phonopy, i.e., the supercell method, and there is no imaginary mode. Dunno if the code interpolates differently or enforces the acoustic sum rule in a different way, but it is weird that the collaborator uses phonopy with QE and gets positive modes and I don't... Thanks again for the hint! Thomas -- Dr. rer. nat. Thomas Brumme Theoretical chemistry TU Dresden - BAR / II49 Helmholtzstr. 18 01069 Dresden Tel: +49 (0)351 463 40844 email: thomas.bru...@tu-dresden.de ________________________________ Von: users <users-boun...@lists.quantum-espresso.org> im Auftrag von Vahid Askarpour <vh261...@dal.ca> Gesendet: Mittwoch, 14. Juli 2021 17:29 An: Quantum ESPRESSO users Forum Betreff: Re: [QE-users] Interpolation for 2D phonons results in one imaginary branch near Gamma Hi Thomas, This may not be directly related to your question and you may have already tried it but I found that for 2D materials, if in the vc-relax step, I ensure that the in-plane stresses are almost or exactly zero, I can make the flexural mode energies entirely positive along the various BZ segments. Cheers, Vahid Vahid Askarpour Department of physics and atmospheric science Dalhousie University Halifax, NS CANADA > On Jul 14, 2021, at 12:00 PM, Thomas Brumme <tbru...@msx.tu-dresden.de> wrote: > > CAUTION: The Sender of this email is not from within Dalhousie. > > Did anyone try recently the 2D cutoff technique for phonons? Is there > maybe an error in my input? > > Regards > > Thomas > > On 7/7/21 11:37 AM, Stefano Baroni wrote: >> Oh, certo: scusa, dimenticavo. S. >> >> -- >> Stefano Baroni, Trieste -- swift message written and sent on the go >> >>> Il giorno 7 lug 2021, alle ore 10:42, Thomas Brumme >>> <tbru...@msx.tu-dresden.de> ha scritto: >>> >>> Dear all, >>> >>> I think I don't see the wood for the trees and hope that someone can help. >>> >>> I want to calculate the phonons for a 2D material using the 2D cutoff >>> technique. The scf calculations for the well-relaxed system work without a >>> problem and I also get no error calculating the phonons on a 5x5x1 q-point >>> grid. The relevant portions of the scf and phonon input are attached below. >>> All frequencies are positive at every single q point, except the 3 acoustic >>> branches at the Gamma point which are "only" -10 cm^-1 and -15 cm^-1. >>> However, the interpolation, in order to get the dispersion, results in one >>> branch which is negative for quite some portion near Gamma - for the first >>> half along G-K I have up to -130 cm^-1 and along G-M up to -60 cm^-1. I >>> guess that this is the out-of-plane acoustic branch and now I wonder if >>> this is real (i.e., the system is really unstable) or if I make some >>> obvious mistake. I will also check by explicitly calculating few q points >>> in the relevant region but before I wanted to ask here. >>> >>> Any help would be appreciated! >>> >>> Regards >>> >>> Thomas Brumme >>> >>> -------------------------------------------- >>> scf: >>> >>> &control >>> calculation = 'scf' >>> prefix = 'pasc12', >>> pseudo_dir = '/home/tbrumme/pseudos/pslibrary.1.0.0/rel-pbe/', >>> outdir = '/scratch/ws/0/tbrumme-pasc/', >>> verbosity = 'high', >>> wf_collect = .true., >>> / >>> >>> &system >>> ibrav = 0, >>> nat = 14, >>> ntyp = 3, >>> ecutwfc = 70.0, >>> ecutrho = 560, >>> noncolin = .true., >>> lspinorb = .true., >>> vdw_corr = 'DFT-D', >>> nbnd = 96, >>> assume_isolated = '2D', >>> / >>> >>> &electrons >>> conv_thr = 1.D-14, >>> / >>> >>> ATOMIC_SPECIES >>> C 12.0 'C.rel-pbe-n-kjpaw_psl.1.0.0.UPF' >>> P 30.973761 'P.rel-pbe-n-kjpaw_psl.1.0.0.UPF' >>> As 74.921595 'As.rel-pbe-dn-kjpaw_psl.1.0.0.UPF' >>> >>> ATOMIC_POSITIONS crystal >>> ... >>> >>> CELL_PARAMETERS angstrom >>> 6.69668 0.0 0.0 >>> -3.348340 5.799495 0.0 >>> 0.0 0.0 15.0 >>> >>> K_POINTS automatic >>> 25 25 1 0 0 0 >>> >>> -------------------------------------------- >>> phonons: >>> >>> Phonon dispersions for pasc12 >>> &inputph >>> tr2_ph=1.0d-18, >>> prefix='pasc12', >>> ldisp=.true., >>> nq1=5, nq2=5, nq3=1, >>> outdir='/scratch/ws/0/tbrumme-pasc/', >>> fildyn='pasc12.dyn.xml', >>> max_seconds = 83700, >>> verbosity='high', >>> recover=.true. >>> alpha_mix(1)=0.4, >>> / >>> >>> -------------------------------------------- >>> q2r and matdyn: >>> >>> &input >>> fildyn='pasc12.dyn.xml', flfrc='pasc12.551.fc', loto_2d=.false., >>> zasr='simple', >>> / >>> >>> &input >>> flfrc='pasc12.551.fc.xml', flfrq='pasc12.freq', q_in_band_form=.true., >>> loto_2d=.false., asr='simple', >>> fldyn='pasc12.dyn.xml' >>> / >>> 4 >>> 0.00000000 0.00000000 0.0 60 >>> 0.33333333 0.57735027 0.0 30 >>> 0.00000000 0.57735027 0.0 45 >>> 0.00000000 0.00000000 0.0 1 >>> >>> -- >>> Dr. rer. nat. Thomas Brumme >>> Theoretical chemistry >>> TU Dresden - BAR / II49 >>> Helmholtzstr. 18 >>> 01069 Dresden >>> >>> Tel: +49 (0)351 463 40844 >>> >>> email: thomas.bru...@tu-dresden.de >>> >>> _______________________________________________ >>> Quantum ESPRESSO is supported by MaX >>> (www.max-centre.eu<http://www.max-centre.eu>) >>> users mailing list users@lists.quantum-espresso.org >>> https://lists.quantum-espresso.org/mailman/listinfo/users >> _______________________________________________ >> Quantum ESPRESSO is supported by MaX >> (www.max-centre.eu<http://www.max-centre.eu>) >> users mailing list users@lists.quantum-espresso.org >> https://lists.quantum-espresso.org/mailman/listinfo/users > > -- > Dr. rer. nat. Thomas Brumme > Theoretical chemistry > TU Dresden - BAR / II49 > Helmholtzstr. 18 > 01069 Dresden > > Tel: +49 (0)351 463 40844 > > email: thomas.bru...@tu-dresden.de > > _______________________________________________ > Quantum ESPRESSO is supported by MaX > (www.max-centre.eu<http://www.max-centre.eu>) > users mailing list users@lists.quantum-espresso.org > https://lists.quantum-espresso.org/mailman/listinfo/users _______________________________________________ Quantum ESPRESSO is supported by MaX (www.max-centre.eu<http://www.max-centre.eu>) users mailing list users@lists.quantum-espresso.org https://lists.quantum-espresso.org/mailman/listinfo/users
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