[QE-users] Adsorption calculation using Quantum ESPRESSO

[Kazume NISHIDATE]

Dear Akhil,

It looks like a reasonable value. Cl atom will become a -1 charged state
subtracting the electron from the oxygen atoms of Cr2O3.

> However, if we look at the literature, this value normally lies in
>  the positive region, which is why Cr2O3 helps steel resist
>  corrosion due to chlorine attack.

The "positive adsorption energy" means that, in this definition, Cl
atom will not be adsorbed on the Cr2O3. You should carefully check the
definition of adsorption energy in the literature.

> In my calculation, this value is coming negative. If anyone can
> guide me in this regard, I will be really grateful.

The "negative adsorption energy" means that Cl will be adsorbed
on the Cr2O3. It is the energy gain to form the adsorption state.

> The same input with only Cl yields a final energy of -79.2218317877
>  Ry.

We usually use the atomic chemical potential \mu rather than the isolated atomic energy derived in a vacuum to derivate the adsorption energy.

The atomic chemical potential \mu is the per one atom energy when
the atoms form their gas or bulk state (natural condensed state of the atom).

I think you should calculate the energy of a Cl2 molecule in a vacuum (gamma point only),
E(Cl2_mol) as suggested by Giovanni, and you can evaluate the atomic chemical potential of Cl as
\mu_Cl = E(Cl2_mol)/2.

The "charged" effect affects the adsorption formation energy as in the
impurity formation energies in semiconductors. But it is not held for your case since the total charge of the supercell would be zero.


> 2024/08/31 4:33、Akhil g.nair via users <users at lists.quantum-espresso.org>のメール:
> 
> The Cl is an isolated atom as these calculation involve chlorine in the ground water which is mostly in the form of NaCl.
> 
> But in the literature, they have used an isolated Cl atom.
> 
> Is there a way to assign charge to atoms as the isolated atom should be in the form of Cl-?
> 
> 
> On Friday 30 August, 2024 at 10:51:28 pm IST, Giovanni Cantele <giovanni.cantele at spin.cnr.it> wrote:
> 
> 
> 1) I would check the convergence of all calculations. In particular you use paw pseudos but ecutrhp = 4*ecutwfc. Values of 8 up to 12 times ecutwfc might be required
> 2) Did you calculate the energy of Cl as that of the isolated atom or as half that of the isolated Cl2 molecule that should be how Cl is found in gas form? What about this energy in your reference papers? Indeed, Cl2 should result more stable thus giving to a larger formation energy in your formula
> 
> Giovanni


西館数芽
Kazume NISHIDATE Ph.D

Department of Systems Innovation Engineering,
Graduate School of Science and Engineering, Iwate University
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kazume.nishidate at gmail.com, nisidate at iwate-u.ac.jp
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