Properties of the surface region of a metal crystal

Total-energy calculations from first principles have been made on a seven-a tomic-layer slab of Mo(001) as a function of the in-plane lattice parameter with full relaxation of the layer spacings. The energy minimum gives the e quilibrium state of the slab, which contracts both in plane and out of plan e between one and two per cent with respect to bulk. The energy changes und er deformation from equilibrium are treated as strain energies and are fitt ed to a composite elastic model consisting of two surface regions and a bul k region, each with its structural and elastic parameters. These parameters are evaluated in a separate calculation for the bulk region, so that subtr acting the known bulk strain energies from the total strain energy permits evaluation of the parameters of the surface region. Six deformations of the slab around equilibrium give the six elastic constants of the tetragonal s urface regions. The surface region material is about two atomic layers deep , slightly prolate in its own equilibrium state, substantially elastically anisotropic compared to cubic symmetry, stable, but considerably weaker ela stically and closer to instability than bulk.