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

[Ari P Seitsonen]

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 / Ari.P.Seitsonen at iki.fi / 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
> 
> 
> 
>