[Pw_forum] contradictory results of electron-phonon interaction for different k-grids

[高淼]

Dear all pw users:

I'm using QE-5.0.1 to calculate electron-phonon interaction for a ternary system (ibrav=4) with Li, B, and C.
But I get contradictory results for different k-grids.

Calculation details:
I choose ecutwfc =40.0, ecutrho=480.0, combined with smearing = 'gaussian', degauss = 0.02 Ry.
The lattice parameters have been relaxed for different k-grid and pseudo potentials to get
the corresponding equilibrium point. For example, the lattice parameters for pbe US potentials
are about a=2.84 angstrom  and c=7.12 angstrom. I set conv_thr =  1.0d-14 and tr2_ph=1.0d-18
in the scf and phonon calculation, respectively.

I use three different groups of k-grids to test the calculated results.
the first group:
densh  k-grid: 24x24x12
coarse k-grid: 12x12x6
           q-grid: 8x8x4
the second group:
densh  k-grid: 32x32x16
coarse k-grid: 16x16x8
           q-grid: 8x8x4
the third group:
densh  k-grid: 36x36x24
coarse k-grid: 18x18x12
           q-grid: 6x6x4


Different pseudo potentials are also tested.

PBE:
Li  6.941   Li.pbe-s-van_ak.UPF
C   12.011  C.pbe-van_ak.UPF
B   10.81   B.pbe-n-van_ak.UPF

PW91:
Li  6.941   Li.pw91-s-van_ak.UPF
C   12.011  C.pw91-van_ak.UPF
B   10.81   B.pw91-n-van.UPF

PZ (combined NCPP and USPP):
Li  6.941   Li.pz-s-van_ak.UPF
C   12.011  C.pz-van_ak.UPF
B   10.81   B.pz-bhs.UPF

PZ-vbc:
Li  6.941   Li.pz-n-vbc.UPF
C   12.011  C.pz-vbc.UPF
B   10.81   B.pz-vbc.UPF

Input file for electron-phonon interaction:

 &inputph
  tr2_ph=1.0d-18,
  prefix='LiBC',
  fildvscf='LiBCdv',
  amass(1)=6.941,
  amass(2)=12.011,
  amass(3)=10.81
  outdir='/home/gm/work/tmp/',
  fildyn='LiBC.dyn',
  electron_phonon='interpolated'
  trans=.true.,
  ldisp=.true.
  nq1=6, nq2=6, nq3=4

My questions are as follows.

(1)The electron-phonon interaction parameter lambda is extremely large for the lowest phonon
branch at some q points. For example for pbe-USPP, the largest lambda for the first group is 2.1725
at q = (    0.000000000  -0.577350269   0.099769209 ), for second group is 19.4839 at
q = (    0.125000000   0.360843918   0.000000000 ), and for third group is 3.2192 at
q = (    0.000000000  -0.577350269   0.099718425 ). And the similar behaviors for PZ-NCPP.
Is the extremely large lambda for lowest phonon branch reasonable?
(2)for the third group k-grid with PBE-USPP, when asr=simple, there is a small imaginary
frequency about -8 cm^-1 around Gamma to A (0,0,pi/c). But when asr=crystal, there is no
imaginary frequency along the high symmetry lines. While for PZ-PP, when asr=simple,
there is no imaginary frequency along the high symmetry lines. So which result should I trust?
(3) For the second group, after applid asr, there are lots of imaginary frequencies, with the
maximum about -167 cm^-1 at q=(0.186667  0.323316  0.199453). The result contradict that of
the third group. But the first group k-grid show similar behavior with the third group,
why does the second group k-grid show different results?

Could anyone give me some clue? I am greatly indebted to you for your help. Thanks again!

Miao Gao
------------------
Ph.D. student of Department of Physics,
Renmin University of China.
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