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

[Thomas Brumme]

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
>>
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-- 
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