Artem, Good results for graphene without any k-point sampling (only Gamma point)? And also for a zigzag NR? Now THAT's weird... unless you are using a huge supercell for each of them. What is your criterion for saying that your calculation is good? For a zigzag NR you might get away with a smaller supercell, if it's a semiconductor (I guess it is, I don't remember it right now).
What is your procedure for calculating the band structure? Are you making sure you re-use the converged DM file? Marcos On Thu, Apr 22, 2010 at 8:06 PM, Artem Baskin <[email protected]> wrote: > 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 > > > >
