Electric field

[Dipankar Saha]

Hello,

For a bulk structure, can we apply vertical electric field  (in out of the plane direction) ?

Best_
Dipankar

[Petr Khomyakov]

For bulk configuration, you can apply an electric field as described in this tutorial at https://docs.quantumwise.com/tutorials/opening_a_band_gap/opening_a_band_gap.html. If you want to apply an electric field across a truly-3D bulk crystal (no vacuum) in a given crystallographic direction, at the moment there is no feature in QuantumATK to do it.

[Dipankar Saha]

Okay, I understand. Thank you Petr ! 

[Petr Khomyakov]

I forgot to mention that you can also induce an effective electric field across a given structure by applying a finite bias voltage using the NEGF approach (i.e., using device configuration). The structure, which is supposed to be exposed to the electric field, should then be sandwiched between two semi-infinite electrodes instead of vacuum. 

[Dipankar Saha]

I forgot to mention that you can also induce an effective electric field across a given structure by applying a finite bias voltage using the NEGF approach (i.e., using device configuration).

Yeah, I know that. I was not talking about NEGF. Instead of making a device structure, if we need to see the effect of field (in out of the plane direction), then what will be the way out?

Perhaps, the tutorial you mentioned could be useful. 

Thanks and Regards_
Dipankar Saha

[Dipankar Saha]

1) Can it be emulated with "external_potential = AtomicCompensationCharge"?

2) What is 'Solvent Di electrical Constant'?
 

[Petr Khomyakov]

1) Can it be emulated with "external_potential = AtomicCompensationCharge"?

No.

2) What is 'Solvent Di electrical Constant'?

"The dielectric constant of the media surrounding the configuration", as described in the manual, https://docs.quantumwise.com/manual/Types/DirectSolver/DirectSolver.html.

[Dipankar Saha]

1) Can it be emulated with "external_potential = AtomicCompensationCharge"?

No.

Please provide some details.......

Besides, it has been mentioned_

"No attention was paid to the interlayer distance. You can change it a bit to see how the band structure changes. Note, however, that you cannot use DFT/GGA to optimize this distance because no energy minimum will be found, due to the absence of dispersion corrections (the layers are bound by Van der Waals forces, not captured in standard GGA). LDA works, fortuitously, because of a cancellation of errors, but the results cannot really be trusted."

What does this really mean? Is it the interlayer distance between two graphene layers? How about the vacuum layers in out of the plane directions (which separate the metallic regions form the actual system) ?

Best_
Dipankar

[Petr Khomyakov]

If you want to introduce an external field, you may do it with metallic spacial regions as described in the tutorial.  I am not aware of an option that uses AtomicCompensationCharge for that.

"No attention was paid to the interlayer distance. You can change it a bit to see how the band structure changes. Note, however, that you cannot use DFT/GGA to optimize this distance because no energy minimum will be found, due to the absence of dispersion corrections (the layers are bound by Van der Waals forces, not captured in standard GGA). LDA works, fortuitously, because of a cancellation of errors, but the results cannot really be trusted."

What does this really mean? Is it the interlayer distance between two graphene layers? How about the vacuum layers in out of the plane directions (which separate the metallic regions form the actual system) ?

- It means that neither GGA nor LDA accounts for van der Waals interaction between two graphene monolayers. This interaction determines the separation distance between the two monolayers, meaning that it should be included when doing geometry optimization. In QuantumATK, this can be done using, the Grimme corrections https://docs.quantumwise.com/tutorials/dispersion_corrections_and_bsse/dispersion_corrections_and_bsse.html.

- There is no van der Waals or any other interaction (through the layer of vacuum) between the graphene monolayers and the metallic regions. The separation distance between them is a free parameter, i.e., it is not optimized while doing geometry optimization.   

[Dipankar Saha]

So you say, if Grimme DFT-D2 has to be included, that should be done only for the vdW stacking part.
Correct?

The separation distance between them is a free parameter, i.e., it is not optimized while doing geometry optimization.   

How does this separation distance (large / or, small) matter?

[Petr Khomyakov]

I do not know if the correction should really be to turned off (in practical calculations) for the rest of the system, which is not a layered stack. But that might work, since the dispersion forces are usually essential only for the layered stack when it comes to geometry optimization. Actually, it also depends on the system, since there are 3D materials (e.g., some oxides as far as I remember) where the van der Waals forces are important as well.

Regarding setting the vacuum thickness, it is pretty much up to you, as there is no chemical interaction between the metal regions and the actual atomic structure - it is pure electrostatics determined by the Poisson solver with particular boundary conditions imposed on the metallic regions. Your system looks like Dirichlet (or Neumann) boundary|vacuum|Stack|vacuum| Dirichlet (or Neumann) boundary, where the two vacuum regions act as dielectric spacers (with epsilon=1) in which the electric field will be constant.

[Dipankar Saha]

Okay.... 
Many thanks for all the information Petr !!

Best_
Dipankar