[Pw_forum] input file for transmission

[Manoj Srivastava]

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)

&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. 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, 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?

Regards, 
Manoj Srivastava
Ph.D. student
Department of Physics
University of Florida, Gainesville, FL