I am at the beginning of my PhD and I just learned about the dipole correction in ab-initio calculations. I have seen that it is used mainly when an electric field is applied to a system. But I have also read in https://christoph-wolf.at/2018/05/02/dipole-correction-in-quantum-espresso/ that some surfaces have non-vanishing dipole density. I wonder if that is the case for TMDs and if the ferroelectric dipole in some of them makes necessary this correction to obtain accurate results.
1 Answer
$\begingroup$
$\endgroup$
I will try to explain why we need dipole corrections using the example of adsorbed atom or molecule on a surface. Suppose that we want to calculate the adsorption energy of an adatom on the surface of monolayer TMD or any other surface as you can see in the figure:
In this case, we have created a dipole in our structure, and this will affect the potential energy at the vacuum level. Typically, you want the potential to be as accurate as possible. When you have the vacuum, you should have a flat potential energy surface or something near flat. When potential is no longer flat at vacuum, you have effectively a solitude potential that was induced by the introduction of the adatom.
Despite that you are not applying an electric field to the structure, you are still getting an effect that simulates an electric field, and this is not ideal and could distort the electron density of the slab and change the energy, that's where you need to correct this perturbation with the dipole correction. The following figure will show you how the potential energy without dipole correction is not longer flat at the vacuum level
Suggestion:
I suggest you to calculate the electrostatic potential of the bilayer structure and see whether the second layer is leading to a non-flat potential at the vacuum level, in this case a dipole-correction is needed. Depending on the code you are using, as mentioned in the link you have shared, QE uses emaxpos, eopreg and eamp. If you are using VASP, then the dipole correction is controlled via LDIPOL and IDIPOL tags, check the documentation for further read.
