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Dear users,

I want to get the total energy of ionized molecules like O2 anion, something like that. I used the IDIPOLE = 4 to correct the monopole and dipole interactions. I thought that the corrected energy should be lower than the energy without correction because the repulsive monopole-monopole interaction is much larger than the attractive dipole-dipole interaction. But I got the opposite result. Actually, all the correction energies for the monopole-monopole and dipole-dipole had plus values. Can you explain me the reason why the energy correction for monopole-monopole interaction is also larger than zero like that of dipole-dipole interaction?

Thank you.


<span class='smallblacktext'>[ Edited ]</span>

The total energy of a charged periodic system is infinite. The uncorrected total energy is not infinity, and is equivalent to a charged system neutralized electrostatically by a jellium. For the case of O2 anion (negatively charged), the self-consistent potential should be pushed up relative to the neutral system. However, for a plane-wave code, the average of the self-consistent potential is always set to be the reference zero. The overall result is that there is over-screening by the jellium, and a cell-size dependence of the total energy. The monopole correction tries to correct the electrostatic energy because of how the plane-wave code define the vacuum level. You can refer to the VASP manual and the cited references within for details.