Hello QE users,
I am trying to calculate the phonon properties of different point defects in
monolayer (2D) hBN. As a test calculation, I am considering the neutral VnNb
defect (nitrogen vacancy with a neighboring boron replaced by a nitrogen). For
testing purposes, I am considering a small 17-atom unit cell with 15 A of
vacuum.
In order to obtain a relaxed structure, it appears that I cannot enforce
cell_dofree='ibrav+2Dxy' and instead use cell_dofree='2Dxy'. The resulting
forces acting on each atom are zero up to and including the sixth decimal
plane. The pw.x output states that the stresses are 0.00000000 Ry/bhor^3 for
each component, expect for the zz component (0.00000086 Ry/bohr^3), which is
treated using assume_isolated='2D'.
My issue is that ph.x calculates negative frequencies for the first two modes
(-50 cm^{-1} and -12 cm^{-1}). It doesn't appear to be an issue related to
negative frequencies at the Gamma point, because these two modes have very
little dispersion across the Brillouin zone. I have performed a similar
calculation for pristine hBN (also using ibrav=0 to verify this isn't the
issue) and obtain all positive frequencies. Am I treating the 2D slab
correctly? Any thoughts on why the phonon code indicates there is instability?
pw.x input:
&CONTROL
calculation = 'scf'
restart_mode = 'from_scratch'
prefix = 'hBN'
pseudo_dir = './pseudo/'
outdir = './TMP_DIR/'
etot_conv_thr = 1.0E-12
tstress = .true.
tprnfor = .true.
/
&SYSTEM
ibrav = 0,
nat = 17,
ntyp = 2,
ecutwfc = 50.0
ecutrho = 600.0
occupations = 'smearing'
smearing = 'm-p'
degauss = 0.0002
assume_isolated = '2D'
nosym = .true.
/
&ELECTRONS
diagonalization = 'cg'
conv_thr = 1.0E-14
/
&IONS
ion_dynamics="bfgs"
/
ATOMIC_SPECIES
B 10.811 B.pbe-n-kjpaw_psl.1.0.0.UPF
N 14.0067 N.pbe-n-kjpaw_psl.1.0.0.UPF
ATOMIC_POSITIONS (crystal)
B 0.1236294548 0.2288517946 0.5000000000
B 0.1088156910 0.5613303716 0.5000000000
B 0.1072329417 0.8927673484 0.5000000000
B 0.4864819943 0.2526124675 0.5000000000
N 0.4361682614 0.5638317166 0.5000000000
B 0.4386698609 0.8911844944 0.5000000000
B 0.7855708745 0.2144293085 0.5000000000
B 0.7473878305 0.5135182681 0.5000000000
B 0.7711482933 0.8763705944 0.5000000000
N 0.2442446869 0.1315370037 0.5000000000
N 0.2243740862 0.4579580637 0.5000000000
N 0.2160185581 0.7839816944 0.5000000000
N 0.5616626235 0.1119836884 0.5000000000
N 0.5420420278 0.7756259377 0.5000000000
N 0.8946390297 0.1053610363 0.5000000000
N 0.8880165315 0.4383375889 0.5000000000
N 0.8684632701 0.7557555456 0.5000000000
K_POINTS automatic
2 2 2 0 0 0
CELL_PARAMETERS angstrom
7.591938503 -0.231222917 0.000000000
-3.995637427 6.459556319 0.000000000
0.000000000 0.000000000 15.000000000
ph.x input:
Phonon dispersions for hBN
&inputph
prefix = 'hBN'
outdir = './TMP_DIR/1',
fildvscf = 'dvscf'
fildyn = 'hBN.dyn'
tr2_ph = 1.0d-16
recover=.true.,
alpha_mix(1) = 0.1
search_sym = .true.
amass(1)=10.811,
amass(2)=14.0067,
ldisp=.true.,
nq1=2, nq2=2, nq3=2,
start_q=1, last_q=1
/
Thanks,
Ken
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