[QE-users] Interpolation for 2D phonons results in one imaginary branch near Gamma

[Vahid Askarpour]

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 <tbrumme at 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 <tbrumme at 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.brumme at tu-dresden.de
>>> 
>>> _______________________________________________
>>> Quantum ESPRESSO is supported by MaX (www.max-centre.eu)
>>> users mailing list users at lists.quantum-espresso.org
>>> https://lists.quantum-espresso.org/mailman/listinfo/users
>> _______________________________________________
>> Quantum ESPRESSO is supported by MaX (www.max-centre.eu)
>> users mailing list users at 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.brumme at tu-dresden.de
> 
> _______________________________________________
> Quantum ESPRESSO is supported by MaX (www.max-centre.eu)
> users mailing list users at lists.quantum-espresso.org
> https://lists.quantum-espresso.org/mailman/listinfo/users