[Pw_forum] Pw_forum Digest, Vol 24, Issue 30

[ambavale sagar]

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} 
When doing calcuIation with electrode infinite in xy plane, is it necessary to use same k-pt grid for conductance and scf calculation?
When doing PDOS calculation of molecule, is the gamma point calculation sufficient?

Thanking you.

Sagar Ambavale

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Message: 1
Date: Mon, 15 Jun 2009 09:04:03 +0200
From: Gabriele Sclauzero <sclauzer at sissa.it>
Subject: Re: [Pw_forum] homo-lumo and conductance
To: PWSCF Forum <pw_forum at pwscf.org>
Message-ID: <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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