About the NELECT tag and its formulation

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alpinnovianus
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About the NELECT tag and its formulation

#1 Post by alpinnovianus » Tue Nov 16, 2021 2:40 am

Dear all,

I am interested in using the NELECT tag to perform some hole/electron doping studies in a bulk material.
In VASP wiki, it is mentioned that when we add/reduce the number of electrons with this feature, then a compensating background charge is also added.
I think it is a convenient feature, but I am curious about the formulation behind this tag.

Especially, I read in the past posts in this forum that using NELECT is only an approximation.
Therefore, I hope to understand this tag better in terms of the actual formulations to know how to properly use it and also to know when I can/cannot rely on this feature.

Does anyone know if there were papers or presentations published that explain the NELECT feature in VASP in more detail ?

Also, I will be grateful if somebody could point me to some published papers that exemplify an acceptable usage of NELECT tag in their research on doping systems.

Thank you very much.

henrique_miranda
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Re: About the NELECT tag and its formulation

#2 Post by henrique_miranda » Tue Nov 16, 2021 10:11 pm

I moved this question to this forum that is better suited for this type of question.

NELECT adds a certain number of electrons per unit cell to your system.
This is useful to perform computations on for example doped semi-conductors or charged defects for example.

For what kind of computations do you intend to use the NELECT tag for?
If you are interested in the latter you can for example have a look at this thread:
https://www.vasp.at/forum/viewtopic.php?t=5407

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Re: About the NELECT tag and its formulation

#3 Post by alpinnovianus » Wed Nov 17, 2021 2:30 am

I moved this question to this forum that is better suited for this type of question.
Thank you for approving the post and relocating it to a more suitable forum.
NELECT adds a certain number of electrons per unit cell to your system.
This is useful to perform computations on for example doped semi-conductors or charged defects for example.

For what kind of computations do you intend to use the NELECT tag for?
If you are interested in the latter you can for example have a look at this thread:
https://www.vasp.at/forum/viewtopic.php?t=5407
I want to reduce the electrons to simulate various doping levels in doped copper oxides, so it is not the same case as the one in that thread.

As I read in the past posts in this forum that using NELECT is only an approximation, I would like to understand it better.
If there are similar examples, or if there are documentations on how the compensating background charge is introduced, I hope you could share it with me.

Thank you.
Alpin

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Re: About the NELECT tag and its formulation

#4 Post by henrique_miranda » Wed Nov 17, 2021 7:09 am

As I read in the past posts in this forum that using NELECT is only an approximation, I would like to understand it better.
Could you link the thread here for future reference?

To understand how the compensating background charge is added I recommend the documentation here:
https://www.vasp.at/wiki/index.php/Mono ... orrections

And these two papers:
https://doi.org/10.1103/PhysRevB.51.4014
https://doi.org/10.1103/PhysRevB.78.235104

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Re: About the NELECT tag and its formulation

#5 Post by alpinnovianus » Wed Nov 17, 2021 1:31 pm

Could you link the thread here for future reference?
https://www.vasp.at/forum/viewtopic.php?f=4&t=18280

(on second thought, I wasn't certain whether the "crude approximation" comment by martin.schlipf refers to NELECT usage or the "look at the band structure without setting NELECT and reason about where the electrons/holes would go" or both)

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Re: About the NELECT tag and its formulation

#6 Post by henrique_miranda » Wed Nov 17, 2021 7:50 pm

Ok, thanks for posting the link.
Very likely he was referring to the latter.

Also in practice, doping in a semiconductor is created by adding impurities.
As such, if you want to simulate the doping by a certain type of impurity you would create a supercell of your bulk material with an impurity inside.
The size of the supercell of bulk material should be chosen to represent the carrier concentration you are interested in.

In this sense, using NELECT on the pure system is an approximation because what you would like to have is... a system with impurities.
But there is a limit to the size of the supercell that you can compute in practice so a limit to impurity concentrations you can simulate (it's hard to simulate low impurity concentrations).
You also know that the fewer impurities there are the more the material looks like a perfect bulk.
So NELECT just gives you a way to compute properties on a material where the only thing that changes is the number of electrons in the unit cell (no impurities added).

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