You don't have to take more than 1 k-point in z direction. I got also confused by these examples. A more detailed explanation can be found in departments.icmab.es/leem/siesta/tlv14/slides13.pdf page 5 and 17.
You have to use a high number of k-points in transport direction for the electrodes, 1 k-point in transport direction for the device and also TBTrans, because of the open boundary condition. In the transverse direction however, you have to use the converged k-point from the siesta runs in the electrode and device, and a much higher number in the TBTrans run. Best wishes, Frank On 11/12/2016 05:03 PM, Zara Nosh wrote: > Dear Dr. Thomas Archer > Thank you very much for your reply. > > I completely agree with you, but in all transiesta examples > <http://dipc.ehu.es/frederiksen/tstutorial/index.php/Exercises_TS_School_2009>, > They have taken high k-point in transport direction for device and > also tbtran run. > As we have open boundary condition in z direction, I can't understand > why we should take more than 1 k-point in z direction. > > Any comments are appreciate. > Best > Zara > > On Sun, Nov 6, 2016 at 9:32 PM, Thomas Archer <[email protected] > <mailto:[email protected]>> wrote: > > I have little knowledge of trans siesta but I work on smeagol, the > methodology should be the same. > > > > K points are a little different than standard DFT, we typically > use several steps so avoid a massive self consistent cycle with a > dense K-point grid. > > > electrodes: These are constructed periodic along z so should be > constructed with k-points in the z direction. You want to get the > electrodes as accurate as possible since this is one of the least > expensive parts of the calculation you should use lots of K-points > along z. The xy plane should be dense enough to give the correct > electronic structure similar to a standard DFT > calculation. (e.g.4x4x100) > > > SCF of the junction: Since you break the symmetry along Z when > you construct the junction it makes no sense to use more than 1 > k-point along z. In the xy plane use a normal number of > k-points (for siesta) and this should be kept consistent for the > electrodes and the junction. (e.g.4x4x1) > > > > Transmission: Here the xy plane you should have a very dense grid > as the transmission is very k-dependant, but typically does not > need to be done self consistently. (e.g. 100x100,1) > > > Tom Archer > > > > > ------------------------------------------------------------------------ > *From:* [email protected] <mailto:[email protected]> > <[email protected] <mailto:[email protected]>> on behalf > of Zara Nosh <[email protected] <mailto:[email protected]>> > *Sent:* Sunday, November 6, 2016 5:20:35 PM > *To:* [email protected] <mailto:[email protected]> > *Subject:* [SIESTA-L] k-point sampling in transiesta > > Dear all, > For transiesta calculations, we have to use high k-point in the > transport direction (z) to simulate the semi-infinite electrodes. > > Do we need high kz, just for electrodes' calculations? or for > both electrodes and device? > > Can we use normal kz (like usual siesta calculations) for device > part? For example if our system is large in z direction, about 60 > Ang, can we use just 1 k point for kz? > > I really appreciate your help in advance. > With best regards, > -Zara > Tern uni > >
