[Pw_forum] Graphene Nanoribbon (Negative phonon frequencies and structure relaxation)

[Prabhakar Marepalli]

Hello everyone,

I'm trying to compute phonon dispersion properties of graphene nanoribbon
using Quantum Espresso. My unit cell is a simple armchair structure with 4
atoms per unit cell. It looks like below:

       0----0
      /       \
     0        0

The above structure is replicated in the y-direction (the periodic
direction) while the x-direction is confined. Using the above structure,
here is the outline of the problems I'm running into:

1) Negative phonon frequencies (not only at gamma point, but at all other
k-points)

2) I tried using 'asr' but that doesn't seem to fix the problem.

3) I understand that the problem might be due to instability of the
structure (I did notice that the forces on atoms in scf calculation is
non-zero , forces of the magnitude 0.2 to 0.4).

4) When I try to relax the structure using 'ion_dynamics', the forces did
go down but the structure is relaxed so much that it is no longer graphene
nanoribbon.

Can you please give me some pointers on how to approach this problem, i.e.
how do I get rid of negative frequencies while relaxing the structure that
constrains it to look like a nanoribbon ?

Thanks for your time!

Here are my input files:

pw.x (this doesn't include the ion_dynamics relaxation).

 &control
    calculation='scf'
    restart_mode='from_scratch',
    tstress = .true.
    tprnfor = .true.
    prefix='gnr_arm',
    pseudo_dir = '/work/02420/pm24229/QE/QE/pseudo/',
    outdir = '/home1/02420/pm24229/tmp/',
 /
 &system
    ibrav=  0, nat=  4, ntyp= 1, nbnd= 20,
    ecutwfc =25.0, occupations='smearing', smearing='marzari-vanderbilt',
    degauss=0.02
 /
 &electrons
    diagonalization='cg'
    conv_thr =  1.0d-12
    mixing_beta = 0.7
 /
ATOMIC_SPECIES
 C  12.0107  C.pbe-rrkjus.UPF

CELL_PARAMETERS {angstrom}
 2.5546 0.00000000 0.00000000
 0.00000000 10.0 0.00000000
 0.00000000 0.00000000 2.00000000

ATOMIC_POSITIONS {angstrom}
 C 0.0  0.0  0.0
 C 1.2773 0.7375 0.000
 C 1.2773 2.2125 0.00
 C 0.0  2.95 0.0

K_POINTS automatic
20 1 1 1 1 1

ph.x

phonons on kspace
 &inputph
  tr2_ph=1.0d-14,
  prefix='gnr_arm',
  ldisp=.true.,
  nq1=20, nq2=1, nq3=1
  amass(1)=12.0107,
  outdir='/home1/02420/pm24229/tmp/',
  fildyn='gnr_dyn',
 /

Thanks,
Prabhakar Marepalli
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