# [Pw_forum] input file for transmission

Gabriele Sclauzero sclauzer at sissa.it
Wed Apr 15 09:03:11 CEST 2009

```
Manoj Srivastava wrote:
>
> But that is exactly the trouble I am having in this. The scattering
> region should not be periodic like leads, as now we have infinite
> scattering region! But PWSCF always has PBC, so we should have a large 3rd
> lattice vector to make the scattering region practically finite.

But in this way you would have a cluster made of 5Al and one hydrogen, with the Al atoms
at the edges disconnected from the leads. You want the charge density at the edge atoms to
be as similar as possible to that of the leads (in this case an infinite monatomic chain).

Even if the scattering region is computed as a periodic system with PBCs, the KS potential
inside the cell is not k-dependent and you can use it to solve the scattering problem with
different boundary conditions (not periodic).
The k-dependent KS eigenstates obtained from pw.x calculations are not used in the pwcond
calculation, only the V_KS(G) in the (super)-cell is needed (super because it needs to be
larger than the periodicity of the monatomic wire).

> I dont
> see any super cell here. a_3 is just 1.875*a_0, where a_0 is lattice
> constant. The atomic postions are all in a_0 unit, which makes me believe
> that it is a practically infinite system with a_3=1.875*a_0.
>
>>> 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.
>
>  In the above example, in principle we can have one atom H as scattering
> region, and Al wire as left and righ leads, but we have taken few Al atoms
> with H and treated it as scattering region. Thats what I
> meant by how much part of leads should be taken as scattering region.

OK, I got it now. I think I have already replied to this somehow, when speaking about the
convergence criterion. The H impurity perturbs the charge density of the wire (with
respect to the pristine wire configuration, that you take as the lead region) not only on
the Al atoms hosting the hydrogen, but also on further atoms.
You have to include in the scattering region as many Al atoms as needed, such that the
perturbation on the charge density induced by H on the edge atoms is negligible (you can
check this from the CBS, as pointed out in my previous reply).
In this way the charge density will not change abruptly when crossing the border between
the lead and the scattering region and the KS potential will not have discontinuities.

GS

--

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

```