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Is the introduction of a vacuum in the z-axis necessary when utilizing a 2D monolayer material for Density Functional Theory (DFT) calculations in Quantum Espresso? (This is due to the fact that only one layer is utilized for the calculations, and there are no inter-layer interactions in the z-axis ?)

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As your system will be only periodic in 2D, yes, you need to add a vacuum layer (in both sides of your layer) to avoid image interactions.

The ideal way to do this is running a convergence test where you calculate the system energy using different vacuum layer size and then make a graph energy vs vacuum layer. From this graph you can select a good value for the vacuum layer when the energy stop changing (depending on the vacuum layer size).

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  • $\begingroup$ Thank you for answering my question, @Camps. But there is one unclear point to me, Could you please explain how it would be possible for there to be image interaction in the z direction if I am only using and interested in monolayers in my calculations? $\endgroup$
    – Thejan Hasaranga
    Commented Apr 11 at 1:48
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    $\begingroup$ Since in principle Quantum Espresso assumes periodicity in all three directions, your unit cell is periodically repeated in every direction through boundary conditions. The vacuum layer acts as a "buffer" of sort that makes sure that the periodically repeated images in z-direction are far enough apart to eliminate unwanted interaction and give you a monolayer. $\endgroup$
    – manju9
    Commented Apr 11 at 7:18

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