Dear Prabhakar,
Ehmm, I might be wrong, but isn't this a zig-zag ribbon that your input
produces (vertical lines for unit cell; the vertical-to-horisontal ratio
not correct)::
|
| |
C C C C C C C <-- edge
c c c c c c c
... c c c c c c c ...
C C C C C C C <-- edge
| |
The arm-chair would be
| |
|CC |CC CC <-- edge
... cc cc cc cc ...
|CC |CC CC <-- edge
|
Then two issues: The zig-zag ribbon is spin-polarised
(anti-ferromagnetic, the opposite edges with opposite spins) and now that
I start to remember it, it might be that indeed this ribbon turns into two
linear chains (five+ years since those calculations by Tobias...); please
try with a wider one (or indeed the arm-chair ribbon).
Greetings,
apsi
PS Please remember to attach your affiliation to your mails
-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-
Ari Paavo Seitsonen / [email protected] / http://www.iki.fi/~apsi/
Ecole Normale Supérieure (ENS), Département de Chimie, Paris
Mobile (F) : +33 789 37 24 25 (CH) : +41 79 71 90 935
On Fri, 2 Jan 2015, Prabhakar Marepalli wrote:
Dear Dr. Seitsonen,
Thanks for checking my script. You are right that the structure I posted runs
in x-direction, but the problems I mentioned are exactly the same for this
script too (I used y-notation as it is
easier to draw and explain the unitcell that way). Also, thanks for pointing
about the unitcell plane distance in z-direction. I've done the following
changes:
1) I've increased the lattice constant in z-direction to a very large value of
10 Angstroms, but it didn't change my answers by a lot.
2) My structure is periodic in all directions, but I've picked the lattice
vectors in y,z directions to be very long that the structure essentially acts
like a 1D.
3) I've added the 'ecutrho' parameter and picked a value of ecutrho=12*ecutwfc.
This hasn't changed my answers either.
For relaxing, I've made the following changes to my original script:
&control
calculation='relax'
&ions
ion_dynamics='bfgs'
&cell
cell_dynamics='bfgs'
press_conv_thr=0.4,
cell_factor=3.0
The above relaxation almost changed my nanoribbon lattice to a square lattice.
Can you please suggest if I'm using any of those parameters incorrectly?
Thanks,
Prabhakar Marepalli
On Fri, Jan 2, 2015 at 1:16 PM, Ari P Seitsonen <[email protected]> wrote:
Dear Prabhakar,
Did you check your structure? To me it looks very weird in 'XCrysDen',
the planes of the periodic GNR's along z direction only 2 Angstroms apart. And
yes, you should relax the
structure before calculating phonons. And your system is periodic in all
directions, and the GNR seems to run along x, not y like I understood from your
description below. Please also
read the instructions about 'ecutrho' when using ultra-soft pseudo
potentials or the PAW method.
Good Luck, :)
apsi
-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-
Ari Paavo Seitsonen / [email protected] / http://www.iki.fi/~apsi/
Ecole Normale Supérieure (ENS), Département de Chimie, Paris
Mobile (F) : +33 789 37 24 25 (CH) : +41 79 71 90 935
On Fri, 2 Jan 2015, Prabhakar Marepalli wrote:
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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