Neither textbooks like Kittel or Ashcroft&Mermin nor information gathered in under/graduate courses on solid state physics, considering quasi-continuum quasi-momenta/wavevectors provide much support on efficient k-sampling using very modest k-grids. I was asking for some practical hints on reasonably k-sampling (presumably much more similar to small-crystal approaches (http://link.aps.org/doi/10.1103/PhysRevB.37.6073, http://link.aps.org/doi/10.1103/PhysRevB.44.8554), not obvious for people with background in general solid state physics but with very little experience in tran/siesta-like calculations; a point certainly known for experts in numerical approaches to nanotransport.
Concerning the main text of the PCCP-paper mentioned: 1. I can only find that five k-points were used in electronic structure calculations (I presume kx=ky=5 used for siesta relaxation) and 25< k_xy<50 for "transport calculations". It is not clear to me whether "transport calculations" refer to the section transiesta < SR.fdf > SR.out or to the section "tbtrans < SR.fdf > tbt-SR.out & 2. Sorry for having possibly overlooked something, but I cannot find any information on the kz-values used in all these runs. What I see in the ESI is an overall use of kz=1. This kz-value is highly puzzling for me. On Tue, Jan 19, 2016 at 10:01 PM, Nick Papior <[email protected]> wrote: > You seem to think that k-points are a generic chosen value. It is not. > You should think of k-points in _exactly_ the same way as in regular > siesta calculations. > And what does one do with k-points in siesta? > Basically, you converge them... > > I would highly advice you to read Kittel, Martin, or any other book on > condensed matter theory where the Brillouin zone is explained! > Perhaps, that would reveal the importance of choosing a correct and > appropriate k-point sampling? > If you have a professor, ask her/him for guidance. > > NOTE: > http://www.rsc.org/suppdata/c5/cp/c5cp04613k/c5cp04613k1.txt > the k-point samplings for transiesta and tbtrans _are_ different. > REMARK: that this input fdf file is for a future release of > transiesta/tbtrans. Hence not all keywords are used in the currently > available transiesta/tbtrans. > The k-grid is as written in the article. 5 during transiesta, 25 and 50 > for tbtrans. The value 25 also exists in that file. > > > > 2016-01-19 21:38 GMT+01:00 ticu Cubot <[email protected]>: > >> Dear TranSiesta users, >> >> I am (still) confused on the k-sampling needed in the three steps >> (denoted 1, 2, 3 below) in transport calculations for molecular junctions. >> Let me refer to Au-BDT-Au, with all BDT atoms having y=0 (or almost zero). >> Is is correct, if I (roughly) set >> >> 1. kx=8, ky=2, kz=50 for electrode-alone calculations to get the files >> electrode.TSHS >> > kx and ky should be the same as chosen for your device calculation. > kz should just be very high. I tend to use 100, but anything above 50 > would probably suffice... (converge if in doubt...) > >> >> 2. ky=8, ky=2, kz=2 the SR-transiesta step to obtain the SR- files >> (SR.TSDE, SR.TSHS) needed for tbtrans. kx and ky same as in step 1, ky < kx >> because of BDT extension along x in transverse direction. In this choice, I >> was (correctly?) inspired by the k-values chosen in >> > Converge kx and ky. Understand what they imply by reading more material. > >> >> http://www.rsc.org/suppdata/c5/cp/c5cp04613k/c5cp04613k1.txt >> >> 3. kx=16, ky=4, kz=50 for tbtrans calculations. That is, more dense >> sampling for tbtrans than for transiesta (step 2), because >> k_xyz-convergence of transmission is slower. Here I would note that in the >> above link the same kgridMonkhorstPack seemed to have been used both in >> step 2 and 3 (this is very surprising for me!); anyway, >> TBTkgridMonkhorstPack is not explicitly given, and because other TBT >> settings are given in that file, I assume that transiesta-k-grid and >> tbtrans-k-grid were the same. >> > Converge kx and ky. Set kz=1. > > >> >> Many thanks for illuminating me on this point. >> >> ticu >> > > > > -- > Kind regards Nick >
