[Pw_forum] Why is different bewteen example12 of PWSCF and test08 of WanT?

[Andrea Ferretti]

On Thu, 19 Jan 2006 yuss at jlu.edu.cn wrote:

> Dear all:
>      The example12 of PWSCF and test08 of WanT all study the transport of Al chain. 
>      Why are results ( trans.alwire and cond.dat) different?
>      Thanks! 

Hi,

concerning the ideal aluminum chain,
example12 in espresso and test08 in WanT do not have the same geometry 
(mainly, different bond length)... 

according to me, there is also a factor 2 (due to a different convention 
in the choice of the transmitance units): espresso results must be divided 
by 2 to be in the same units of WanT 

once these problems are overcome, a discrepancy is still around.
this is due to a more physical reason: when computing the 
transmittance across the scattering (conductor) region, the matrix green 
function technique implemented in WanT treats the leads as a chain of 
principal layers with nearest neighbour interactions.

these layers can be made larger and larger in order to control the 
convergence of the calculation, but in the present case the aluminum 
chain in WanT was simulated with one atom per unit cell 
(= the principal layer, here), 
making the difference wrt example12 in espresso...

Since it is quite nice to have such a direct comparison with different 
methods, I update test08 in WanT in order to be the most similar to 
example12 as possible...

You can find this updated version at

http://nanofs.unimo.it/~ferretti/test08.tar.gz  

the calculation now includes:
*   Al chain with one atom per cell (as before)
*   Al chain with 5 atoms per cell (now directly comp with example12)
*   Al chain with H impurity       (also comparable with example12)

results from the two methods are now in a pretty  good agreement
the README file in the test dir gives more details about the 
comparison

thanks for the tip
Andrea