I want to study interface between water and metal. So I take unit cell and add 20A of vacuum and a water molecule on c-direction. My slab will be repeated in 3 dimensions and periodicity will appear in a- and b- dimensions. But why some researchers are using large values of a and b, despite it significantly increase computational time? For example not just 1x1 MAPlI3, but 2x2 MAPlI3. Is there difference between repetition of 1x1 cell and 2x2 cell?
Thank you in advance!
If you're running your calculations in VASP, because of the periodic boundary condition, the metal slab which you consider will be repeated in all the 3 directions, and hence you cannot adsorb the water molecule ( or any adsorbant species)on the active sites of the catalyst. To avoid this, you leave "x" distance of vacuum. The vacuum is chosen such that the bottom layer of the upper unit cell doesn't have an effect on the adsorbant species. I hope I'm clear enough to you!
Narasimhan Viswanathan, thank you for your answer! Unfortunately, I use QE, but i think it is not important. I have unit cell, with a x b x c parameters. And I added 20A of vacuum and a water molecule on c-direction. As you said, my slab will be repeated in 3 dimensions and periodicity will appear in a- and b- dimensions. But why some researchers are using large values of a and b, despite it significantly increase computational time? (For example not just 1x1 MAPlI3, but 2x2 MAPlI3).
I edited my question, to make it more clear.