The electrostatic interaction of an external charged system with a metal surface: a simplified density functional theory approach

As a first step to meet the challenge to calculate the electronic structure and total energy of charged states of atoms and molecules adsorbed on ultrathin insulating films supported by a metallic substrate using density functional theory (DFT), we have developed a simplified new DFT scheme that only describes the electrostatic interaction of an external charged system with a metal surface. This purely electrostatic interaction is obtained from the assumption that the electron densities of the two fragments (charged system and metal surface) are non-overlapping and by neglecting non-local exchange-correlation effects such as the van der Waals interactions between the two fragments. In addition, the response of the metal surface to the electrostatic potential from the charged system is treated to linear order, whereas the charged system is treated fully within DFT. In particular, we consider the classical perfect conductor model for the metal response, although our formalism is not limited to this approximation. To test the computational implementation of this new scheme, we have considered the case of a Na+ cation interacting with a perfect conductor. The application of this new methodology to realistic problems involving charged systems adsorbed on insulating films supported by a metal surface are deferred to a separate following publication.