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I am doing molecule calculations where I am trying to figure out the value of the electrostatic potential of the vacuum. For neutral molecules (or slabs) a dipole correction should suffice and the electrostatic potential of the vacuum can be obtained far away from the molecule if the cell is sufficiently large.

Now, for charged molecules one needs to introduce the neutralizing background to overcome the infinite energy due to interacting charged periodic replicas, and the monopole correction for the potential should also be added.

Doing so, I have noticed that the potential far away from the charged molecule decays exponentially as V(r) = A + B * exp(-r/C). Can this A be considered the vacuum level? In a real situation, the potential should decay as V(r) = A + B/r, so that the bare electrostatic potential of the excess charge would be retrieved.

My questions are:
1) Why is the calculated potential screened and how should it be interpreted? Does this depend on the choice of functional?
2) How can the vacuum potential be extracted from charged molecule calculations in VASP?

Any input would be greatly appreciated.

Miguel

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