construct Au electrode with 2 × 2 layers in x and y directions

[J.g_johnson]

Dear friends,
   I am trying to construct Au electrodes with  2 × 2 layers in x and y directions and eight slabs in Z direction. As i don`t know the experimental parameter of Au electrode, so i have tried a lot of times to optimize the structure of Au electrode. No matter what i did, the results were not what i want. So if you could give me some directions or help. I would very appreciate it.

[Umberto Martinez]

How do you optimize the structure?

You should first optimize the Au bulk structure (optimize the lattice parameter) and only after this step you create the 2x2 Au surface with 8 layers.
Add the structure form the Database.
You can then optimize the lattice parameter by removing the "constrain cell" option in the OptimizeGeometry block.
Remember to check the convergence with respect to k-points sampling.

[J.g_johnson]

     Thank you for your guide.
   You mean in the Builder, I should add a Gold structure(FCC) from the Database. Then Press the "send to" button in the lower righter corner of Builder and select the Script Generator.

    In the Script Generator, add a New Calculator and a Optimize Geometry block to optimize the Gold structure.
    In the New Calculater, the number of K-points is 3*3*50.
    In the Optimize Geometry block, I remove the tick from the cell constraint in X,Y and Z direction, and the stress tolerance to 0.001eV/Å.
    Then the script is send to the Job Manager.
 
    I have done it as above. But the results of the structure show that there is no change at all.
   And what should I do next? Repeat the optimized Gold structure in X,Y and Z direction? I am not sure. The attached picture shows the electrodes I want.
  

[Umberto Martinez]

The steps you follow above are correct and are done in order to optimize the Au bulk structure.
Just use a mesh of k-points like (n,n,n) because the unit cell is cubic (around 20x20x20 should be more then fine)

The second step is to build the device configuration as you report in your picture.
Sorry if my last reply was not complete but I may misunderstood your initial question.

1- take the optimized bulk structure and send it to the Builder.
2- here, with the Builder>Surface(cleave) plugin cut a (100) or a (111) surface according to your wish.
From your figure it looks like the plane perpendicular to the transport direction is the (100)..
Here, you can also define the unit cell (you mentioned 2x2)
Always with this plugin, choose the number of layers you want.
3-  Once the structure is created, transform you bulk configuration in a DeviceConfiguration using the Device Tools>Device from Bulk plugin.

In step 2 and 3 choose carefully the number of layers that will constitute the central region and the electrodes.
here some relevant informations:
http://www.quantumwise.com/documents/tutorials/latest/ATKTutorialDevice/index.html/chap.input.html#sect2.input.devicefrombulk
http://quantumwise.com/publications/tutorials/mini-tutorials/99

[J.g_johnson]

Thank you, Mr Martinez. You`ve maken a very clear guidance. thank you, again!
I think I have made some improvements by follwing your guide and the figure1 shows the structure I got.

Then I made some trials to construct a whole two probe model including both left and right parts. But I am not sure whether the processes are reasonable.
After getting the figure1, the structure was repeated in C direaction 2 times, using the "repeat" plugin in Bulk Tools. So there are 4 layers in C direction.
Then I "drag" repeated structure itself into the builder. After that,  Z`=Z+10 was inputted into Transform by Expression plugin in the Coordinate Tools to seperated them.
Then using Device Tools->Device From Bulk, so there is a two probe with right and left electrodes, as figure3.

But the  "Screening Region" is 4 layers which may be slightly more. And I am not sure whether the whole processes are reasonable.

[Umberto Martinez]

yes, the process is reasonable.
Indeed you may have too few layers. Please check again the two links above for some more information.

A faster and easier way could be to use the Builder->Interface plugin using the structure you used to create your last figure 1 twice.
With the tool Shift Surfaces inside this plugin you can increase the Z separation.
Finally simply use Add/Remove Layer to get exactly what you want.

[J.g_johnson]

     Thank you, Mr Martinez. I think I now know how to deal with it and I will check again with the two links above. Thanks a lot.