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Hi is it possible to take two +ve charged ions in a unit cell with large vacuum around? and how do I do that, because for neutral atoms normally we just give the position in POSCAR and Potentials are taken from POTCAR file. IN case of +ve charged ions Do we need to do any modification in INCAR/POTCAR and what?

as vasp is not based on local basis functions, charges cannot be assigned to individual atoms at all, additional charges can only be given for the unit cell as a whole. The calculation will converge to its self-consistent equilibrium charge density, one can check a posteriori (by plotting the charge density difference between the charged and the neutral unit cell) where the additional charge has piled up.