We present a practical scheme for performing ab initio supercell calculations of charged slabs at constant electron chemical potential , rather than at constant number of electrons N e . To this end, we define the chemical potential relative to a plane ͑or "reference electrode"͒ at a finite distance from the slab ͑the distance should reflect the particular geometry of the situation being modeled͒. To avoid a net charge in the supercell, and thus make possible a standard supercell calculation,

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... restore the electroneutrality of the periodically repeated unit by means of a compensating charge, whose contribution to the total energy and potential is subtracted afterwards. The "constant " mode enables one to perform supercell calculation on slabs, where the slab is kept at a fixed potential relative to the reference electrode. We expect this to be useful in modeling many experimental situations, especially in electro-chemistry.