[Pw_forum] input file for transmission

Gabriele Sclauzero sclauzer at sissa.it
Tue Apr 14 09:35:03 CEST 2009

Manoj Srivastava wrote:
> Dear PWSCF users and developers, 
>  I wish to do a transmission calculation and confused about the input
> file. I have a question on example 12 of the package, where transmission
> of monoatomic Al wire with a H atom adsorbed on the side is done. The SCF
> run in the device region is done with (some part of input file is given
> below)

it is usually called the "scattering region"

> &system
>     ibrav = 6,
>     celldm(1) =12.0,
>     celldm(3) =1.875,
> where the atomic postions of different atoms is 
> Al     0.00000000     0.00000000     0.0000
> Al     0.00000000     0.00000000     0.375
> Al    -0.02779870     0.00000000     .75537515
> H      0.19269012     0.00000000     .93750000
> Al    -0.02779870     0.00000000     1.11962485
> Al     0.00000000     0.00000000     1.5
> So, looking at the z coordinate of above system, we notice that device
> region is periodic with period 1.875. 

You are right, the scattering region is a periodic system, since pwscf always uses PBCs. 
The lenght of the scattering region id 12.0*1.875 a.u. and contains all 5 Al and the H 

> So, physically we are solving for an
> infinite device region, but in the physical setting of a transmission
> problem leads are semi-infinite and device is finite. Shouldn't we use
> some kind of vacum, i.e. taking 3rd lattice vector large, which
> effectively would represent the finite device region? 

> Also how much part
> of the leads should be taken as part of device region, 

I don't understand this point. The leads are conceptually different thing than the 
scattering region. The lead is a periodic unit of the "bulk" region (in this case an 
infinitely long monatomic wire) and it is used to compute the generalized Bloch states, 
which in turn are propagated in the scattering region.

> Is there some kind
> of convergence criterion? Is it like keep increasing part of lead in the
> device reion till further increase does not substantial change device
> behavior, e.g. Bloch's state?

There is a main convergence criterion (though I don't understand if you are actually 
refering to this). You have to increase the scattering region, adding more Al atoms in the 
wire, such that the complex band structure with real wave-vectors computed using the 
leftmost periodic unit of the wire included in the supercell (the H impurity being in the 
middle of the s.c.) converges to the band structure of an impurity-free wire, obtained for 
instance from a pwscf calculation (or from a pwcond calculation with a 1 atom cell 
containing an Al atom).

To do this you can use pwcond with
prefixt='prefix of the scattering region'
bdl=ratio between the lenght of the periodic unit and celldm(1)
band_file='name of file containing the CBS'

then compare the real bands (contained in <band_file>.re) with those from pwscf (obtained 
using the 1 atom cell).

Also convergence of the transmission with the lenght of the scattering region can be used, 
but it is quite more cheap to check convergence of CBS (which can also help to understand 
if everything is going fine), and when the CBS of your "bulk" region (leads) is correctly 
reproduced the transmission should be converged as well.



> Regards, 
> Manoj Srivastava
> Ph.D. student
> Department of Physics
> University of Florida, Gainesville, FL
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> Pw_forum at pwscf.org
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| Gabriele Sclauzero, PhD Student                  |
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