1ST-PRINCIPLES CALCULATION OF THE GEOMETRIC AND ELECTRONIC-STRUCTURE OF THE BE(0001) SURFACE

Linearized-augmented-plane-wave calculations for a nine-layer Be(0001) slab agree with the unusual experimental finding of a substantial outer-layer expansion relative to the truncated bulk lattice. They imply that the separation between the outer two layers should be 3.9% larger than in the bulk, while the second- to third-layer separation should be 2.2% larger. The surface expansion is accompanied by demotion of p(sigma) to s electrons on outer-layer Be's. The surface Be's loss of three neighbors makes the energy cost of s- to p(sigma)-electron promotion, which is necessary for the formation of strong bonds to the next layer down, less profitable than in the bulk.