Beryllium-hydrogen ultrathin films. II. Ground-state properties of the dilayer

The thinnest possible realization of H over Be, the heterogeneous dilayer (Be 1-layer and H 1-layer) has three possible H positions of high symmetry relative to the Be 1-layer (1L) with Be(0001) symmetry: open, overhead, and bridge. Of these, we predict the open site to be energetically favored, with an equilibrium interplanar distance c=2.02±0.02 a.u., and intraplanar distance ${\mathit{a}}_{\mathit{h}}$=4.01±0.02 a.u. All three conformations are predicted to be metallic. The total energies, uniaxial energy second derivatives, and Kohn-Sham energy bands, as determined by all-electron, full-potential, local-density-functional calculations are also reported. Comparison with previous extended and cluster-model calculations on H over Be shows that Be-H 2L has the same site ordering (open preferred over bridge over overhead) as the ${\mathrm{Be}}_{36}$H cluster but not the same as the 1L-4L-1L supercell (bridge preferred over open over overhead). Extended-system and large-cluster models give calculated values of ${\mathit{a}}_{\mathrm{BeH}}$ that are grouped closely. The work function of the 2L is a strong function (a volt or more) of site configuration. For the preferred open site, it is 0.9 eV below the value for the bare Be 1L and 0.6 eV below the bulk Be experimental value.