[Pw_forum] homo-lumo and conductance
Gabriele Sclauzero
sclauzer at sissa.it
Mon Jun 15 09:04:03 CEST 2009
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>
> |
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> | skype: gurlonotturno |
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| 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 |
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