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 >> >> >> >> > _______________________________________________ > Pw_forum mailing list > [email protected] > http://pwscf.org/mailman/listinfo/pw_forum >
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