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

[Thomas Brumme]

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