Dear Marcos, Thanks for your help, but the situation looks really WEIRD or I don't understand anything at all (that may be also true). When I was calculating the BS for graphene and zigzag NR I did not use any specific supercell (I did not use the supercell block, the naive supercell factors were 6*6*1 for graphene (initially my elementary unit cell consisted of 2 atoms) and 6*1*1 for Zigzag NR (with 22 atoms - 5 rings width)). Of course, the SCF procedure converged in these two cases. As to criterion to determine whether my calculations are good or not, I used the comparison both with tight-bonding method results and the DFT calculations that were already done (using Siesta) by Pr. Stephan Roche and co-workers. My results tallied with the those ones perfectly.
As to convergence in case with the zigzag NR (with k-point grid), double unit cell of armchair NR and its flakes, the procedure did not converged, and I don't know how to have it converged. Moreover, form one step of the iteration to another one the charges on the edge H-atoms are fluctuating getting more and more asymmetrical. This is another manifestation of the non-zero dipole moment. Best, Artem Baskin, PhD student, University of Illinois at Chicago On Thu, April 22, 2010 1:16 pm, Marcos Veríssimo Alves wrote: > 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 <abas...@uic.edu> 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 >> >> >> >> > >
