  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 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 (PC) model for the metal response, although our
formalism is not limited to this approximation. The successful computational
implementation of this new methodology and the PC model is exemplified by the
case of a Na$^{+}$ cation outside a metal surface.
