2016-01-20 14:39 GMT+01:00 ticu Cubot <[email protected]>: > 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. > As with many other parameters in DFT/DFT+NEGF, you should do convergence tests, also of k-points. This also goes for transport calculations!
Probably nobody can give you practical hints on reasonably k-sampling (this is not entirely true, but...). Hence asking for "a reasonable kgrid" is very dependent on what you wish to examine and hence I, for one, am reluctant to give practical advice. If you start a new study, I would advice you to do convergence tests! Further, to get a feel, and better understanding of their influence I would highly advice you to start with siesta calculations. Bulk calculations! Once you know how "regular" siesta works, then you would also get a better transition to start transiesta calculations. > > 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 & > No, that is not what we write we do. Transport calculation == tbtrans, NEGF/electronic structure calculation == transiesta in this regard. Hint: graphene is only periodic in one transverse direction 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. > I would highly recommend you to reread the transiesta paper. Hint: The answer lies in the self-energies. > > > 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 >> > > -- Kind regards Nick
