ambavale sagar wrote: > Dear Gabriele, > Thank you for your reply. I refered to smogunov paper but I could not > understand the sentence used there : "Since the system has the supercell > (artificial) 2D periodicity in the xy plane, perpendicular to the > transport direction, we averaged the transmission over the corresponding > 2D BZ using 21 k points in the irreducible part of the BZ.." Are those > 21 k-pts belong to xy plane only? what type of grid that may be? 6 6 2 1 > 1 1 ? > In my case supercell dimensions in x and y are 25 a.u. i.e. ~12 ang > which includes nearly ~9 ang. of vaccum.
Here Alexander already replied you. So your k_perpendicular points sampling depends if you want to study an isolated wire (thick or monoatomic), or a wire/molecule between realistic electrodes (modeled as surfaces or surfaces+tips attached to them). In the former case, if your supercell is large enough in the directions perpendicular to the transport the transmission should not depend on k_perp and you need only one k_perp-point. In the latter you need to sample accurately the 2D BZ, since the transmission depends (a lot in some cases) on k_perp. > > As you mentioned the limitation of DFT due to xc-functional, can I use > DFT calculation to compare qualitatively the Transmission coefficient of > same molecule with different geometries? I cannot say, actually. It depends on the coupling and other details. You can have a look to the recent work by Thygesen and Rubio, which try to include a GW correction in this kind of calculations. > > would you please mention in detail how to find positions of HOMO, LUMO > in a coupled system? What I would do (but fur sure there are many better methods) if I want to put, say, O_2 between electrodes is to look at the PDOS (computed by projwfc.x in the QE suite) projected onto the O atoms and compare it with the PDOS of the isolated O_2. It would give an idea of the level of hybridization between molecule and electrodes and of the position of the HOMO(LUMO)-derived levels. It could also be useful to look at the PDOS on the electrode atoms to understand which metallic states/atoms are more involved in the coupling. regards, GS > > regards > Sagar Ambavale > ------------------------------------------------------------------------ > ** > Message: 1 > Date: Fri, 12 Jun 2009 18:14:46 +0200 > From: Gabriele Sclauzero <sclauzer at sissa.it <mailto:sclauzer at sissa.it>> > Subject: Re: [Pw_forum] homo-lumo and conductance > To: PWSCF Forum <pw_forum at pwscf.org <mailto:pw_forum at pwscf.org>> > Message-ID: <4A327EF6.7020206 at sissa.it <mailto:4A327EF6.7020206 at > sissa.it>> > Content-Type: text/plain; charset=ISO-8859-1; format=flowed > > > ambavale sagar wrote: > > Dear all, > > I am studying ballistic conductance through a molecule attached to Al > > electrode. > > Including Al surfaces on both sides? > > > The k-pt grid used for scf calculation of scattering region > > made of molecule and few layers of electrode, is 2 2 10 1 1 1. > > If so, this k-point grid looks a bit lousy to me. Unless you use a very > large supercell in > the transverse directions (xy, since z is the transport direction in > pwcond), you should > take more k-points along that directions. Along z, instead, you need > less k-points (10 > seems really too much to me). Along z you must include enough slabs > before and after the > junction with the molecule such that the scf potential at the borders of > the cell is close > enough to the scf potential of bulk Al. > > You can have a look at this paper in order to get an idea: > Smogunov et al., Phys. Rev. B 78, 014423 (2008) > > > I want to > > see effect of HOMO-LUMO levels of molecule on transmission. Is it > > sensible to find HOMO-LUMO levels of molecule using separate calculation > > of isolated molecule with gamma point? How about bandstructure that I > > got from 20 k-pt calculation of scattering region periodic in > z-direction? > > I'm not sure I got your questions. Of course if you study the ballistic > conductance > through the molecule in the tunneling regime, the result will be very > sensitive to the > position of the HOMO and LUMO levels of the molecule. Depending on how > strong is the > binding between the molecule and the electrodes the position and width > of these levels > will be modified more or less by the hybridization with the metal states. > In order to find the position of the HOMO and LUMO in the coupled system > you can look at > the PDOS on the atoms of the molecule (if not too complex) and compare > it with the > corresponding PDOS of the isolated molecule. > Another solution might be to build Wannier functions of the HOMO and > LUMO and project onto > them, but I never tried this solution. > > Anyway, you should be careful when trusting DFT results in this kind of > system, since > common XC functionals can mismatch the HOMO-LUMO gap of several eV and > there could also be > problems in getting a reliable alignment with the Fermi level of the > metal. Since in > weakly coupled junctions the conductance is very sensitive to the exact > position of the > molecular levels involved in the tunneling and to the decaying behavior > of their tails, > this issues can lead to errors of 1 or several orders of magnitude. > > > Cheers > > GS > > > > > Thank you. > > > > Sagar Ambavale > > PhD student > > M.S. University of Baroda > > India > > > > > > > o ------------------------------------------------ o > | Gabriele Sclauzero, PhD Student | > | c/o: SISSA & CNR-INFM Democritos, | > | via Beirut 2-4, 34014 Trieste (Italy) | > | email: sclauzer at sissa.it <mailto:sclauzer at sissa.it> > > | > | phone: +39 040 3787 511 | > | skype: gurlonotturno | > o ------------------------------------------------ o > > > *************************** > > ------------------------------------------------------------------------ > Own a website.Get an unlimited package.Pay next to nothing.* Click > here!. > <http://in.rd.yahoo.com/tagline_ysb_website/*http://in.business.yahoo.com/> > > > ------------------------------------------------------------------------ > > _______________________________________________ > Pw_forum mailing list > Pw_forum at pwscf.org > http://www.democritos.it/mailman/listinfo/pw_forum -- o ------------------------------------------------ o | Gabriele Sclauzero, PhD Student | | c/o: SISSA & CNR-INFM Democritos, | | via Beirut 2-4, 34014 Trieste (Italy) | | email: sclauzer at sissa.it | | phone: +39 040 3787 511 | | skype: gurlonotturno | o ------------------------------------------------ o
