an arrangement of surface atoms with a slightly lower energy than the ideal arrangement of the bulk solid, and that it involved displacements of the atoms consistent with experimental results. A third example concerns the first stages of the epitaxial growth of nickel silicide on silicon. Calculations indicated that a proposed intermediate site for the nickel atoms was much less favorable than a site corresponding to the final configuration at the fully reacted interface.

It can be anticipated that there will be substantial further growth in the number of groups engaged in total-energy calculations and in the variety of systems to which their results are applied. Although improvement in the basic physical approximations inherent in local-density theory is a long-term goal, the benefits of this method are far from being exhausted. Incremental improvement in computational efficiency can be expected, but trade-offs between accuracy and speed are inescapable. Substantial progress should be made in the new area of isolated adsorbates. The required numerical calculations nearly always consume large amounts of supercomputer time, and it is extremely important that such resources become more widely available. The variety and complexity of the mathematics involved make it improbable that special-purpose computers will be useful in these computations.

These methods are expected to have a growing impact on all areas of surface science and related areas of technology, ranging from semiconductor device processing to catalyst design. There are many more materials for which studies of reconstruction and interface energies will be of value. These calculations can be expected to contribute to the solution of a large number of fundamental problems, such as adsorbate reactions, surface modification, and surface vibrations. Well-designed investigations should lead to a qualitative understanding of mechanisms and chemical trends and not just to the case at hand.

Experimental Probes

It is an unfortunate truism of surface science that surface-sensitive probes must interact strongly with matter and therefore require nontrivial theoretical effort to extract the desired information. One such area that has experienced rapid recent growth is the diffraction of monoenergetic atomic helium beams from surfaces. Though the utility of this method as a structural probe had been limited by the inability to relate the helium-surface interaction potential to surface atom positions, an approximately linear relationship between the repulsive potential acting on the helium atom and the surface charge density has been demonstrated. This quantity can be calculated from surface atom positions by employing simple approximation techniques or by methods related to total-energy calculations. Progress has also been made in the development of better ways to calculate diffraction intensities, which is itself



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