We would like to study the phosphate (H2PO4-) adsorption on Goethite (110) surface. In the experimental study, we have maintained the reaction solution at pH 7 whereas in the theoretical study in order to maintain the same pH value of the simulation cell how many water molecules need to be added?
Hi Babu,
To study these sorts of reactions, typically we don't explicitly simulate pH or potentials in VASP (even though it is possible). In your case, a pH of 7 would require on the order of millions of water molecules for every H+/OH- pair. This is not to mention the inherent difficulties of successfully localizing charge on a molecule like H2PO4-.
Instead, I'd suggest checking out this page for a starting point: https://www.bnl.gov/isd/documents/92710.pdf
That link has a table with the energies of redox couples, including the reduction of the H2PO4 radical species to H2PO4-. This reaction has a standard potential of 2.75 V. The key is that the radical species is easily simulated within VASP. This means that at 0 V vs. SHE, the H2PO4- molecule is 2.75 eV lower in energy than the H2PO4 radical. So you can simply adjust the adsorption energies you determine by this factor.
As for the pH, the pH is easily incorporated by recognizing that the SHE potential decreases by 0.05916 eV for every pH unit (assuming T = 25 degrees Celsius).
You can look up papers that mention the "computational hydrogen electrode" in their methodology... this should explain how pH is factored into the free energy calculations fairly well.
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