General expressions for the density-density and potential-potential response functions, ground-state (i.e., surface) energy, and one-electron optical potential are derived for a model of planar interfaces between two media, each of which is described by a local frequency-dependent dielectric function. These expressions are utilized to evaluate the surface energies characteristic of interfaces between semiconductors (insulators) described by the uniform dielectic function ${\ensuremath{\epsilon}}_{S}(\ensuremath{\omega})=1+{\ensuremath{\omega}}_{p}^{2}{({\ensuremath{\Delta}}^{2}\ensuremath{-}{\ensuremath{\omega}}^{2}\ensuremath{-}\frac{i\ensuremath{\omega}}{\ensuremath{\tau}})}^{\ensuremath{-}1}$ metals described by ${\ensuremath{\epsilon}}_{M}(\ensuremath{\omega})=1\ensuremath{-}\frac{{\ensuremath{\omega}}_{p}^{2}}{\ensuremath{\omega}(\ensuremath{\omega}+\frac{i}{\ensuremath{\tau}})}$, and the vacuum ${\ensuremath{\epsilon}}_{V}(\ensuremath{\omega})\ensuremath{\equiv}1$. Plasmon damping (i.e., nonzero ${\ensuremath{\tau}}^{\ensuremath{-}1}$) is shown to limit the range of nonlocality of the one-electron optical potential to $\ensuremath{\lambda}\ensuremath{\sim}{(\frac{2\ensuremath{\hbar}\ensuremath{\tau}}{m})}^{\frac{1}{2}}$ and to decrease the surface energy. The surface energy of semiconductor interfaces is found to diminish monotonically with increases in the band-gap parameter ${E}_{g}=\ensuremath{\hbar}\ensuremath{\Delta}$. The conventional expressions for the surface energy of metals as a function of their density, $n=\frac{m{\ensuremath{\omega}}_{p}^{2}}{4\ensuremath{\pi}{e}^{2}}$, are recovered in the $\ensuremath{\tau}\ensuremath{\rightarrow}\ensuremath{\infty}$ limit, although errors in some previous derivations of these expressions are displayed. Finally, the structure and limitations of local models of surface properties are examined critically, and the well-known hydrodynamic and step-density random-phase-approximation models of metal-vacuum interfaces are shown to be elementary consequences of classical electrostatics in the limit that ${\ensuremath{\epsilon}}_{M}(\ensuremath{\omega})=1\ensuremath{-}\frac{{\ensuremath{\omega}}_{p}^{2}}{{\ensuremath{\omega}}^{2}}$.