Dear all users, I am involved in band structure calculations for armchair and zigzag nanoribbons and I have problems with the appropriate k-point sampling. My problem is like this: Initially, I calculated the band structure for graphene and zigzag NR without specifying any k-points. The results (with Meshcutoff level 800 Ry and 400 Ry respectively) were quite good. But when I tried to calculate the band structure for armchair (5 rings width) NR I got absolutely wrong results, moreover, in spite of symmetrical configuration I obtained the non-zero significant dipole moment perpendicular to the ribbon (even with 400 Ry MCutoff). Then I generated a k-point grid using %block kgrid_Monkhorst_Pack 45 0 0 0.5 0 1 0 0.5 0 0 1 0.5 %endblock kgrid_Monkhorst_Pack and I got correct results (no dipole moment and reasonable FE and band structure).
FIRST problem that I encountered was that when I tried to recalculate BS for zigzag NR with the SAME k-point grid I got an enormous dipole moment (perpendicular to the ribbon) and wrong BS and FE level. Why is it so? SECOND problem is that when I doubled the elementary cell for 5-rings width armchair ribbon (using the SAME k-point grid and 400 Ry MCoff level) just to make sure that I will get the same good results I got again an enormous Dip moment and incorrect Band Structure. And THIRD (mysterious) problem is that when I tried to calculate Band Structure for an isolated flake of armchair NR (3*5 rings size) I got non zero Dip moment (in this case for x- and y- directions) that is nonsense. I guess, the problem is how to define correctly k-grid in these three cases, but I don't know how. I performed the convergence test increasing the number of k-points but neither dipole moment nor FE level demonstrate the monotonic behaviour, so I could not reach the correct results. Any help will be appreciated. Artem Baskin, PhD student, University of Illinois at Chicago
