layer number of Sceening region

[huixiaodemao0712]

Hi,
 
I'm new in ATK. Following the tutorial(http://quantumwise.com/publications/tutorials/builder/172-building-a-molecular-junction-au-dtb-au#userconsent#), I have learnt how to build a molecular junction.But recently, I was really confused by the layer number of Au atom in the screening region(including electrode extension and surface layers).I want to know:

1.What role do the extension and surface layer serve in the transmission calculation?
2.In the tutorial mentioned above, the screening region has four layers.If my mulecule (only having one benzene ring) is in the simialr size as BDT, will 4 layers in the screening region be OK?
3.Strictly speaking, "The required number of screening layers needs to be checked for convergence"(in tutorial http://quantumwise.com/documents/tutorials/latest/ATKTutorialDevice/index.html/chap.input.html). How to do the convergence experiment in ATK(please give as much detailed instruction as possible )

Thank you for your kind help!

[Anders Blom]

1. In the electrode you have perfect bulk gold (or whatever metal). Closest to the molecule, the gold layers are not bulk gold but surface gold facing a molecule. Somehow you need to make a gradual transition between bulk and surface gold, and that's what the surface layers (screening region) provide.

2. See 3.

3. Increase the screening region and see if the results change qualitatively.

[huixiaodemao0712]

What about the convergence experiment?Could you please tell me how to do it?

[zh]

What about the convergence experiment?Could you please tell me how to do it?

It has been explained in the 3rd point by AB. More detail, do a serials of calculations for different screening layers and plot the transmission spectra as a function of number of screening layers to see the convergence behavior.