Hello, I am working with VASP on free energy calculations for the binding of adsorbates in oxygen evolution reaction on a covalent organic framework (COF) as a surface. I am a beginner in calculations with VASP. Then I would appreciate any help.
I have optimized the cell for my COF, which is a neutral system. Now, I want to optimize an OH- molecule adsorbed on the COF. However, I am unsure how to consider the charge in periodic systems. What keyword should I use in this case?
Thank you very much in advance.
Hi Daniel.
There are a couple of levels of answer to this question.
Firstly, you can *de facto* vary the charge in the system by varying the number of electrons. This is done with the NELECT tag. I believe VASP will use a delocalised background charge to cancel the net moment of this out (otherwise the Coulomb energy of the extra electron interacting with its periodic images tends to infinity, and oh dear ...)
Secondly, It is *not* however clear that adding an extra electron to your system will cause the extra electron to localise on the O (In chemistry-speak, why should the LUMO of the uncharged system be on the OH).
In this way, the "added electron" is delocalised over the entire system by default, and will localise wherever there are minima in the potential energy landscape -- quite likely this would be on your COF rather than the OH, and where it should go is not immediately obvious a priori.
Finally, if you only care about the thermodynamics of the system, you might consider calculating the energy of the *bound* system with an extra electron, and compared that with the neutral COF plus a separate calculation of the OH moiety with an extra electron.
Hope that helps
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