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By definition, the surface energy of a thin film or a slab model is defined as

$$ \sigma = \frac{1}{2A}(E_{slab} - N\cdot E_{bulk}) $$

where $N$ is the number of atoms in the thin film and $E_{bulk}$ corresponds to the average energy of the atom in its bulk phase. I should expect that the surface energy of the slab converge to certain value when I keep increasing the number of layers but what I found is the surface energy of my system increases linearly. I think this is unreasonable. Is there anyone who met this problem before?

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    $\begingroup$ Kindly share more data like POSCAR, INCAR, KPOINT. $\endgroup$ – pranav kumar Mar 26 at 16:57
  • $\begingroup$ What are $E_{slab}$ and $\sigma$ ? $\endgroup$ – taciteloquence Mar 29 at 10:53
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I found the answer already. As the inconsistent calculation between the bulk and the thin film in the DFT calculation process. A small error term will generate a diverge behaviour in the surface energy as I increase the thickness of the slab. One way to fix this issue is to use the data from my thin film calculation to extrapolate the bulk energy without doing an extra isolated calculation on the bulk. Two way was commonly used, they are Linear fitting and Boettger method.

Welcome to discuss this issue and point me out if there is anything unclear.

Reference: Boettger J C 1994 Phys. Rev. B 49 16 798

Fiorentini V and Methfessel M 1996 J. Phys.: Condens. Matter 8 6525

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