Dear QE users,
I've been trying to obtain the phonon dispersion for O passivated Silicon
sheets, and while the relaxation appears correct, and the acoustic modes also
look correct (~ -1 cm-1) at the gamma point , there is always a single mode
with negative frequency away of the gamma point (0.5 0.5 0) with value reaching
~ -200 cm-1.
If I replace the oxygen with 2 H atoms the phonons behave well. Using 2 -OH
instead of 1O also results in the same issue.
I have tried:
Relaxations from 1e-4 to 1e-7
More k-mesh density (up to 12x12x1)
Higher cutoffs: up to 150
DIfferent pseudos (currently using Dojo)
Here is one of the relaxation inputs:
&control
calculation='relax'
prefix='SiO_b_4',
wf_collect = .true.,
pseudo_dir = './pseudos/dojo/',
outdir='./'
etot_conv_thr = 1e-6
forc_conv_thr = 1e-6
restart_mode='restart'
/
&SYSTEM
ibrav = 0
ecutwfc = 120.0
nat = 7
ntyp = 3
assume_isolated = '2D'
nosym = .true.
/
&electrons
conv_thr = 1.0d-10
mixing_mode = 'plain'
mixing_beta = 0.5
diagonalization = 'david'
diago_full_acc = .true.
/
&ions
ion_dynamics = 'bfgs'
/
&CELL
cell_dofree = '2Dxy'
/
CELL_PARAMETERS {angstrom}
3.8 0.0 0.0
0.0 3.8 0.0
0.0 0.0 24.0
ATOMIC_SPECIES
Si 28.08500 Si.upf
H 1.00750 H.upf
O 15.999 O.upf
ATOMIC_POSITIONS {angstrom}
Si 0.0016742746 -0.0713893810 8.1568435063
Si 0.3113041995 1.8307404314 9.4771562615
Si 2.6539898849 3.7328987353 12.0865299573
Si 2.1985890646 1.8307732260 10.8584361966
H -1.4869821232 -0.0709963685 7.9641453095
H 0.5567137638 -0.0718090357 6.7674928212
O 4.1040946046 3.7328817780 12.7505655928
K_POINTS automatic
12 12 1 0 0 0
And the phonon input:
&inputph
tr2_ph=1.0d-18
alpha_mix(1)=0.45
prefix='SiO_b_4'
ldisp=.true.
epsil=.true.
lqdir=.true.
recover = .true.
outdir='./'
fildyn = 'SiO_b_4.dyn.xml'
fildvscf = 'dvscf'
asr=.true.
search_sym=.false.
nq1=6, nq2=6, nq3=1,
/
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