  Supercell models are often used to calculate the electronic structure of
local perturbations from the ideal periodicity in the bulk or on the surface of
a crystal or in wires. When the defect or adsorbent is charged, a jellium
counter charge is applied to maintain overall neutrality, but the interaction
of the artificially repeated charges has to be corrected, both in the total
energy and in the one-electron eigenvalues and eigenstates. This becomes
paramount in slab or wire calculations, where the jellium counter charge may
induce spurious states in the vacuum. We present here a self-consistent
potential correction scheme and provide successful tests of it for bulk and
slab calculations.
