Band gap of Graphene with soc

[Beijixiong]

Dear Vaspers,

I calculated the band gap for graphene with spin-orbit coupling. I noticed that if the kpoint mesh didn't include the k-point where the dirac cone is located, the gap I obtained is smaller than 1meV, which is acceptable. e.g. For primitive cell, kpiont mesh is 11*11*1 gamma centered, the gap is around 0.4meV.

However, if the kpoint mesh include the point where the gap is located, the gap is larger than 1meV, ranging from 5meV to 40 meV depending on the amount of kpoints I used. e.g. for the superlattice cell, I used 11*11*1 gamma centered kpoint mesh, I will obtain a gap around 40 meV. In contrast, if I use 10*10*1 Monkhorst-Pack grids, which does not include gamma point, I can get a gap smaller than 0.1meV. This also happenes to primitive cell, e.g. if I set kpoint mesh to be 33*33*1, gamma centered. This mesh will include K point in Brillouin Zone. The gap will be 5~10 meV.

I am not quite understand this problem because, in my opinion, we should get more accurate gap when we include the kpoint where the gap is located. Could you please tell me what's the reason for that or did I do something wrong?

Some important parameters:
PREC = Accurate
LAGO = FAST
ISMEAR = -5
EDIFF = 1E-5
LSORBIT = True
ISYM = 0

Thanks for your help in advance and I really appreciate that.

[support_vasp]

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