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Let's say I want to work with the slabs, and I want to add 20 Angstrom vacuum space. Do I need just to set lattice parameter c=20 and k-point 1 in this direction?

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If you are using fractional coordinates in your input file, setting c=20, will "zoom" all your cell.

Increasing cell parameters in order to add vacuum will work only if using Cartesian coordinates.

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    $\begingroup$ Thanks for the prompt response. I am talking for the Cartesian coordinates. So my logic is true right for the Cartesian coordinate geometry file? $\endgroup$ – Barix Oct 12 at 20:43
  • $\begingroup$ Yes, If you have a Cartesian coordinate input file, increasing the cell parameters will add a vacuum space. Also, be careful about the type of cell you are using. $\endgroup$ – Camps Oct 12 at 20:47
  • $\begingroup$ But is this 20 here vacuum space? I mean set c lattice parameter or should I add some value to it to call it to vacuum space of 20A $\endgroup$ – Barix Oct 12 at 21:10
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    $\begingroup$ You need to see where your atoms are, v which is the coordinate of the last plane and add the corresponding value in order to get a 20Angs vacuum. $\endgroup$ – Camps Oct 12 at 22:11
  • $\begingroup$ Camps is absolutely right, even if you're working with cartesian coordinates, you cant simply add 20 angstrom to 'c'. You need to look at your unit cell and make sure vacuum is added between periodic images along 'c'. $\endgroup$ – Xivi76 Oct 16 at 2:05

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