[QE-users] Imaginary frequencies away from Gamma point SiO

[Cristian Mauricio Rebolledo Espinoza]

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,
/
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.quantum-espresso.org/pipermail/users/attachments/20240128/a7c94d7e/attachment.html>