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

[Stefano Baroni]

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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