ABINIT Discussion Forums

Dear Abinit users
I am trying to optimize a (3,3) CNT with c=3 (3 times the unit cell size in the z direction). After doing the ecut convergence study, when I did the k point grid convergence, the convergence was not achieving for the simulated CNT, as the ngkpt grid increases, the total energy first increases, then decreases, again increases, then decreases and soon (i.e. fluctuating). The total energy versus k-point grid graph is attached along with this. I am not sure of whether this problem in ngkpt convergence is due to any error in the common input variables used for the CNT in the simulation.
The input file that I used for simulation is as follows:

acell 1 1 1 angstrom
rprim 14.07046406 0.00000000 0.00000000
0.00000000 14.07046406 0.00000000
0.00000000 0.00000000 7.38300513
ntypat 1
typat 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
znucl 6
xred 0.6446 0.5000 0.0000
0.6446 0.5000 0.3333
0.6446 0.5000 0.6667
0.6108 0.5930 0.0000
0.6108 0.5930 0.3333
0.6108 0.5930 0.6667
0.5723 0.6253 0.1667
0.5723 0.6253 0.5000
0.5723 0.6253 0.8333
0.4749 0.6424 0.1667
0.4749 0.6424 0.5000
0.4749 0.6424 0.8333
0.4277 0.6253 0.0000
0.4277 0.6253 0.3333
0.4277 0.6253 0.6667
0.3641 0.5495 0.0000
0.3641 0.5495 0.3333
0.3641 0.5495 0.6667
0.3554 0.5000 0.1667
0.3554 0.5000 0.5000
0.3554 0.5000 0.8333
0.3892 0.4070 0.1667
0.3892 0.4070 0.5000
0.3892 0.4070 0.8333
0.4277 0.3747 0.0000
0.4277 0.3747 0.3333
0.4277 0.3747 0.6667
0.5251 0.3576 0.0000
0.5251 0.3576 0.3333
0.5251 0.3576 0.6667
0.5723 0.3747 0.1667
0.5723 0.3747 0.5000
0.5723 0.3747 0.8333
0.6359 0.4505 0.1667
0.6359 0.4505 0.5000
0.6359 0.4505 0.8333
natom 36
ecut 54
nstep 200
toldfe 5.0d-6
kptopt 1
nshiftk 1
shiftk 0.0 0.0 0.0
ndtset 16
ngkpt1 1 1 3
ngkpt2 1 1 4
ngkpt3 1 1 5
ngkpt4 1 1 6
ngkpt5 1 1 7
ngkpt6 1 1 8
ngkpt7 1 1 9
ngkpt8 1 1 10
ngkpt9 1 1 11
ngkpt10 1 1 12
ngkpt11 1 1 13
ngkpt12 1 1 14
ngkpt13 1 1 15
ngkpt14 1 1 16
ngkpt15 1 1 17
ngkpt16 1 1 18
ixc 11
optforces 1

Can someone guide me of what could be the reason for not achieving the k point grid convergence, how can I modify my input file to get the converged k point grid.

Any help in this regard is greatly appreciated.

Thanks in advance
Seba