Plasmon contribution to surface energies

The surface- and bulk-plasmon contributions to the surface correlation energy are discussed using the classical infinite-barrier model (CIBM). The results are quite different depending on whether we use the random-phase-approximation (RPA) or hydrodynamic approximations. It is argued that relative importance of plasmon contributions exhibited by the CIBM-RPA will be similar to that found in more realistic calculations. It is also shown that the bulk-plasmon continuum contribution is comparable to the surface-plasmon contribution, contrary to what is implied in several recent papers.