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I would like to reproduce the data of adsorption energies published in paper.

For example, they say the adsorption energy for Li atom on graphene is -1.19 eV (estimated by GGA-PBE). I used vasp (5.4) code using parameters indicated in the paper and estimated total energy for

Supercell 4x4x1 of graphene: -0.29529545E+03 eV;
Graphene with one Li atom on its surface: -0.29635605E+03 eV;
One Li atom (estimated from bcc structures): -1.909 eV.

Therefore, the adsorption energy should be -296.35605 eV -[(-295.29545 eV) + (-1.909 eV)] = +0.848 eV. It is inconsistent with the previous calculation.

If I use total energy for the isolated Li atom of -0.29471258 eV, the obtained adsorption energy is -0.767 eV. Both have a big gap with the published data. I wonder if there is a trick to accurately estimate the adsorption energy?

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  • $\begingroup$ Hi, I edited your title to make it more clear. Feel free to roll back or re-edit if it doesn't fit what you are asking. $\endgroup$ – S R Maiti Jun 20 at 12:34
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    $\begingroup$ Does your optimized geometry match the literature one? $\endgroup$ – wzkchem5 Jun 20 at 12:58
  • $\begingroup$ @wzkchem5, I am sorry it is not matched. The result above is on top of C. At the hollow site, it should be -1.104 eV using the total energy of the isolated Li atom. But I don't know when I should use the total energy of one Li atom estimated from bcc structures? And when I should use the total energy of an isolated Li atom? $\endgroup$ – Binh Thien Jun 21 at 1:40
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    $\begingroup$ Adsorption energies of atoms are defined with respect to isolated atoms, unless otherwise specified. I think you should first try to reproduce the literature geometry. If the geometry is wrong, you can never get the energy right. $\endgroup$ – wzkchem5 Jun 21 at 9:38
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    $\begingroup$ @NikeDattani OK, I'll do that $\endgroup$ – wzkchem5 Jun 29 at 12:55
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Following @NikeDattani's advice, now I rewrite my comments into an answer.

First of all, you should try to reproduce the geometry given in the paper. You mentioned in the comment that you got an adsorption energy of -1.104 eV at the hollow site, which is already quite close to the literature value of -1.19 eV. Does the adsorption geometry at the hollow site match the literature perfectly (not only qualitatively but also quantitatively, if you can assess their optimized atomic coordinates)? If the geometry still differs a little bit, then this may be the reason for the remaining discrepancy, for example whether you allowed your lattice constants to relax, whether you used the same slab width with the literature, etc.

As for whether you should use a single Li atom or bulk Li in calculating the adsorption energy, the answer is that unless otherwise specified, you should always use the energy of the single atom. That's how adsorption energy is defined.

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