problems with bandstructure calculations

[Lilia Woods]

Hi,

I have a strange problem with calculating bandstructure of graphene nanoribbon. Here is what I have done and where the problem appears.

- I made a supercell allowing enough distance between the ribbons in each supercell.
- I relaxed the structure using the PBE and LDA options.
- I perform a selfconsistent run to get CHGCAR
- Next, I construct several KPOINTS files with explicit entry of each kpoint. Each file has the k-points for a certain section in the BZ, and since this is a 1d system, only k-points along the dimension of ribbon are non-trivial. Here is the file between 0-0.1 kpoints

k-points
18
Reciprocal
0.000000000000 0.000000000000 0.000000000000 0.028571428571
0.000000000000 0.000000000000 0.005882352941 0.057142857143
0.000000000000 0.000000000000 0.011764705882 0.057142857143
0.000000000000 0.000000000000 0.017647058824 0.057142857143
0.000000000000 0.000000000000 0.023529411765 0.057142857143
0.000000000000 0.000000000000 0.029411764706 0.057142857143
0.000000000000 0.000000000000 0.035294117647 0.057142857143
0.000000000000 0.000000000000 0.041176470588 0.057142857143
0.000000000000 0.000000000000 0.047058823529 0.057142857143
0.000000000000 0.000000000000 0.052941176471 0.057142857143
0.000000000000 0.000000000000 0.058823529412 0.057142857143
0.000000000000 0.000000000000 0.064705882353 0.057142857143
0.000000000000 0.000000000000 0.070588235294 0.057142857143
0.000000000000 0.000000000000 0.076470588235 0.057142857143
0.000000000000 0.000000000000 0.082352941176 0.057142857143
0.000000000000 0.000000000000 0.088235294118 0.057142857143
0.000000000000 0.000000000000 0.094117647059 0.057142857143
0.000000000000 0.000000000000 0.100000000000 0.057142857143

I have similar files for 0.1-0.2; 0.2-0.3, etc sections.

- I obtain the EIGENVAL files from each run and join them.

My problem is the following - the plotted bandstructure look ok only within the first section 0-0.1 of the Brillouiun zone. For the other intervals (above 0.1), there are spiky bands crossing the fermi level. This is not correct since other people have established that the ribbon is semiconducting.

I tried the same thing with the LDA option and got similar results.

The other thing I wanted to mention was that the calculation converged extremely slow for the kpoints larger than 0.1.

Where can the problem be? I tried changing several parameters in the INCAR below but the problem is always there. Here is the INCAR for the bandstructure I used

PREC = High
NELM = 200
NELMIN = 4
ENMAX = 420.00
IBRION = -1
ISMEAR = 1
EDIFF = 0.00001
SIGMA = 0.01
IALGO = 48 algorithm
MAXMIX = 40
LREAL = A
LPLANE = .TRUE.
ICHARG = 11
LORBIT = 1
NPAR = 1
NSIM = 4
RWIGS = 1.640 1.000
NGX = 600
NGY = 432
NGZ = 56

Thanks for any help/comments.

Lilia

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