[QE-users] Phonon dispersion - bad results around K point

[Blair, Erik]

I’m requesting help from all you QE wizards out there.

I’m trying to learn PHonon dispersion calculations, starting simply with diamond. My results look pretty good except for approaching the K point, were I get a bit of a cusp. I’ve tried lots of things: tightening convergences; increasing sampling in the BZ; using asr=’simple’ or asr=’crystal’, etc., but I can’t get rid of the cusping around K. It also makes the X-K path look bad. I’ve tried xc=’lda’ as well as xc=’pbe’.

I have optimized the structure using a PBE ultrasoft pseudo

I point out that I’m using ibrav=2, and I’m using letter notation to specify the high-symmetry points in the path through k-space, so I use q_in_band_form = .true..

Computer/QE version details: I’m using the QE 7.0 package on a Linux HPC cluster.

Relatively low-threshold calculations complete with a bash script and pseudopotentials are available here. I also have my *.out files, and a Python3 script that can generate the plots.

Thanks in advance for your insights.

Enrique (Erik) Blair, Ph.D.
Associate Professor
Electrical and Computer Engineering
Baylor University
Website: http://web.ecs.baylor.edu/faculty/blair/

For your convenience, here’s the LDA work-flow:
# ===== diamond.in =====
&control
  calculation = 'scf',
  restart_mode = 'from_scratch',
  tstress = .true.,
  tprnfor = .true.,
  prefix = 'diamond',
  outdir = 'data',
  pseudo_dir = './',
/

&system
  ibrav = 2,
  nat = 2,
  ntyp = 1,
  a = 3.5689000000000006,
  ecutwfc = 90.0,
  ecutrho = 810.0,
  input_dft = 'lda',
  nbnd = 15,
/

&electrons
  conv_thr = 1.000000d-8,
  mixing_beta = 0.7,
/

ATOMIC_SPECIES
  C    12.011000  C.pz-n-rrkjus_psl.0.1.UPF

ATOMIC_POSITIONS {alat}
  C   0.00000000  0.00000000  0.00000000
  C   0.25000000  0.25000000  0.25000000

K_POINTS {automatic}
    9   9   9   0   0   0

# ===== diamond.ph.in =====
This is a PHonons (ph.x) input file.

&inputph
  tr2_ph = 1.000000d-14,
  ldisp = .true.,
  nq1 = 3,
  nq2 = 3,
  nq3 = 3,
  fildyn = 'diamond.dyn',
  outdir = 'data',
  prefix = 'diamond',
/

# ==== diamond.q2r.in =====
&input
  flfrc = 'diamond.fc',
  zasr = 'crystal',
  fildyn = 'diamond.dyn',
/

# ==== diamond.matdyn.in =====
&input
  asr = 'simple',
  q_in_band_form = .true.,
  flfrc = 'diamond.fc',
  flfrq = 'diamond.frq',
  flvec = 'diamond.modes',
/

  5
gG 40
X 20
K 30
gG 20
L 1

# ===== plot_phonon_dispersion.py =====
import numpy as np
import matplotlib.pyplot as plt

data = np.loadtxt( 'diamond.frq.gp', unpack=True)
fdata = np.delete(data, 0, 0) # delete first row - not needed

markerpos = [0, 40, 60, 90, 110, ]
marker_label = ['$\\Gamma$', 'X', 'K', '$\\Gamma$', 'L']

n_ph, n_qpts = fdata.shape

int_idx = range(0, n_qpts)
for freq in fdata:
   plt.plot(int_idx, freq)

plt.xticks(markerpos, marker_label)
plt.xlim((markerpos[0], markerpos[-1]))
plt.grid(True)

plt.xlabel('$q$')
plt.ylabel('Frequency (cm$^{-1}$)')

plt.tight_layout()

plt.savefig('phonon_dispersion.png')

plt.close('all')


# ===== run.sh =====
#!/bin/bash
mpiexec -n 8 pw.x < diamond.in > diamond.out
mpiexec -n 8 ph.x < diamond.ph.in > diamond.ph.out
mpiexec -n 8 q2r.x < diamond.q2r.in > diamond.q2r.out
mpiexec -n 8 matdyn.x < diamond.matdyn.in > diamond.matdyn.out
python3 < plot_phonon_dispersion.py
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