The nature and strength of the bonding forces between two II-passivated Si surfaces are studied with the density-functional theory, using an approach based on recent theoretical advances in understanding of van der Waals forces between two surfaces. Contrary to previous suggestions of van der Waals attraction between H overlayers, we find that the attraction is mainly due to long-range van der Waals interactions between the Si substrates, while the equilibrium separation is determined by short-range repulsion between occupied Si-H orbitals. Estimated bonding energies and Si-H frequency shifts are in qualitative agreement with experiment. [S0163-1829(98)06448-0].

The applicability of density-functional theory is extended to the area of van der Waals interactions between macroscopic bodies, in particular between two parallel surfaces. It is shown how the strength and the asymptotic form, including the van der Waals planes for the surfaces can be calculated with the electronic densities and the static image planes of the interacting objects as the only input. The calculation is carried out easily in a simple density-functional scheme, suggesting the possibility of extensions to the description of forces between more generally shaped macroscopic bodies, including the sample and tip in scanning-force microscopy.

A general functional form for the exchange-correlation energy of the density-functional theory, valid for widely separated fragments of matter, has recently been proposed [1]. For many kinds of dimers it gives good van der Waals coefficients. Here we also show results for atoms outside surfaces, where it gives a z-3 behavior and reasonable van der Waals coefficients.

A progress report is given of an extension of the density functional formalism to include long-range interactions such as van der Waals or dispersion forces. This is done by proving a general expression for the so-called exchange-correlation energy to contain and to describe such interactions just as well as any other treatment. The proper long-range forms of the interactions are derived explicitly in the cases of two neutral atoms, an atom outside a metal surface, and two parallel metal surfaces. The long-standing problem of treating the attractive and repulsive forces on the same footing in this way gets a solution. For practical calculations, an approximate form, based on an analysis in the weakly inhomogeneous limit and on a limiting form of the three-point function given by Rapcewicz and Ashcroft, is proposed and applied to some prototype cases. (C) 1995 John Wiley & Sons, Inc.