[QE-users] How can I reproduce correct phonon frequencies ?

[Atsushi Yamada]

Hi,

I'm beginner of QE (just I started to use it from a week ago) and I'm
facing a problem that my calculations do not reproduce correct phonon
frequencies. My material is 4H-SiC which is popular and well studied. I
want to reproduce phonon frequencies at gamma point shown in Fig.2(b) of
the following paper:
https://arxiv.org/pdf/1705.02634.pdf

It tells that the phonon dispersion in the figure is calculated by Quantum
Espresso v 5.4.0 and calculation conditions are given. I calculated with
the same conditions, but the high phonon frequencies region are not good
agreement. I tried many calculations with various options and parameters,
but I couldn't get good agreement, and now, I have no idea to solve it. My
resultant frequencies at the gamma point are:

# mode   [cm-1]    [THz]      IR
    1     -0.00   -0.0000    0.0000
    2     -0.00   -0.0000    0.0000
    3      0.00    0.0000    0.0000
    4    192.54    5.7721    0.0000
    5    192.54    5.7721    0.0000
    6    200.76    6.0186    0.0000
    7    200.76    6.0186    0.0000
    8    259.89    7.7914    0.0015
    9    259.89    7.7914    0.0015
   10    404.77   12.1347    0.0000
   11    412.80   12.3755    0.0000
   12    613.53   18.3931    0.1675
   13    785.34   23.5439    0.0320
   14    785.34   23.5439    0.0320
   15    793.13   23.7774    0.0000
   16    793.13   23.7774    0.0000
   17    795.96   23.8622   85.2162
   18    801.09   24.0162    0.0000
   19    801.09   24.0162    0.0000
   20    811.70   24.3343   80.2329
   21    811.70   24.3343   80.2329
   22    851.64   25.5315    0.3206
   23    926.17   27.7658    0.0000
   24    935.31   28.0398    0.0000

Especially, modes 20-24 are obviously different from the paper (and other
literature): The paper is like, (probably) frequencies of the modes 22 and
23 are close (but my result is not like that) and modes 20 and 21 are not
degenerated (but my result shows degeneracy). The results of my
calculations did not change if I use other options, thresholds and
pseudopotential.
Maybe relating to this problem, my cell size obtained by geometry
optimization is not the same to the values shown in the paper (3.07A,
10.05A in Table 1 of the paper).

My input files and calculations are as followings:
(now, I used latest version of QE, 6.4.1, but result by QE 5.4 was the same)

(1) geometry optimization:
     pw.x -in ./SiC.opt.in > ./SiC.opt.out
(2) SCF calculation:
     pw.x -in ./SiC.scf.in > ./SiC.scf.out
(3) Phonon calculation
     ph.x -in ./SiC.phG.in > ./SiC.phG.out
(4) Dynamical matrix calculation
     dynmat.x < SiC.dynmat.in >  SiC.dynmat.out

Input files:
[1] SiC.opt.in
&control
    calculation='vc-relax'
    prefix='SiC',
    pseudo_dir = './pseudo/',
    outdir='./tmp/'
    etot_conv_thr=1d-7
    forc_conv_thr=1d-7
 /
&system
    ibrav=  4,
    A=  3.0760,
    C= 10.0400,
    nat=  8,
    ntyp= 2,
    ecutwfc = 60.0,
    ecutrho =240
 /
&electrons
    conv_thr = 1.0d-10
 /
&ions
    ion_dynamics='bfgs'
 /
&cell
    cell_dofree='ibrav'
 /
ATOMIC_SPECIES
 Si  28.086   Si.pz-n-nc.UPF
 C   12.0107   C.pz-nc.UPF
ATOMIC_POSITIONS angstrom
 Si   0.00000000    1.77592943    2.51000000
 C    0.00000000    1.77592943    4.39250000
 Si   0.00000000    0.00000000    0.00000000
 C    0.00000000    0.00000000    1.88250000
 Si   1.53800000    0.88796471    7.53000000
 C    1.53800000    0.88796471    9.41250000
 Si   0.00000000    0.00000000    5.02000000
 C    0.00000000    0.00000000    6.90250000
K_POINTS automatic
 8 8 4 0 0 0
------------------------

[2] SiC.scf.in
 &control
    calculation='scf'
    prefix='SiC',
    pseudo_dir = './pseudo/',
    outdir='./tmp/'
 /
 &system
    ibrav=  4,
    A= 3.04337366,   !<-- obtained from the optimization
    C= 9.96163727,   !<--  obtained from the optimization
    nat=  8,
    ntyp= 2,
    ecutwfc = 60.0,
    ecutrho =240
 /
 &electrons
    conv_thr = 1.0d-10
 /
ATOMIC_SPECIES
 Si  28.086   Si.pz-n-nc.UPF
 C   12.0107   C.pz-nc.UPF
ATOMIC_POSITIONS angstrom
 Si       0.000000000   1.757092472   2.489491419
 C        0.000000000   1.757092472   4.362543288
 Si       0.000000000  -0.000000000  -0.001538102
 C       -0.000000000  -0.000000000   1.865935344
 Si       1.521686716   0.878546231   7.470309676
 C        1.521686716   0.878546231   9.343361545
 Si      -0.000000000   0.000000000   4.979280155
 C       -0.000000000   0.000000000   6.846753601
K_POINTS automatic
  8 8 4 0 0 0
-------------------------

[3] SiC.phG.in
Phonons of SiC at Gamma
 &inputph
  tr2_ph=1.0d-16,
  prefix='SiC',
  outdir='./tmp'
  fildyn='SiC.dynG',
  epsil=.true.
 /
 0.0 0.0 0.0
-------------------------

[4] SiC.dynmat.in
&input
  fildyn='SiC.dynG',
  asr='simple'
/
------------------------

What is wrong...?
I'm so glad if you kindly help me.

best,
Atsushi Yamada
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