[Pw_forum] homo-lumo and conductance

[Gabriele Sclauzero]

ambavale sagar wrote:
> Sorry for previous mail with wrong subject line.
> 
> Dear Gabriele and Smogunov,
> Yes, I do use Al- wire of finite width as reported in smogunov's 2004 
> paper. In this case what should be the k-pt grid for scf calculation? 
> {for conductance calculation k_perp is set to 1}

For an isolated monoatomic wire or finite cross section electrode (or thick wire) you 
choose kx=ky=1, kz="a number large enough, such that the total energy and other 
interesting physical quantities are converged within your target accuracy".


> When doing calcuIation with electrode infinite in xy plane, is it 
> necessary to use same k-pt grid for conductance and scf calculation?

kx=ky=some number, which needs not to be the same in scf and transmission calculations. In 
the former case you have to converge the total energy, in the latter the transmission (you 
need to check both convergences by increasing kx=ky and looking at the result). Be aware 
that the convergence of the transmission may depend a lot on the scattering energy in some 
cases.

> When doing PDOS calculation of molecule, is the gamma point calculation 
> sufficient?

Yes, if the molecule is isolated.

GS

> 
> Thanking you.
> 
> Sagar Ambavale
> 
> ----------------------------------------------------------------------
> 
> Message: 1
> Date: Mon, 15 Jun 2009 09:04:03 +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: <4A35F263.9020205 at sissa.it <mailto:4A35F263.9020205 at sissa.it>>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
> 
> 
> 
> 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
> 
> 
> 
> 
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| Gabriele Sclauzero, PhD Student                  |
| c/o:   SISSA & CNR-INFM Democritos,              |
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