Density-functional calculation of the bulk and surface geometry of beryllium

The equilibrium lattice parameters and elastic consants of bulk Be and the optimized geometry of the Be (0001) surface were studied using density-functional theory and mixed-basis pseudopotential techniques. Results obtained with various forms of the exchange-correlation functional are compared, including the local-density approximation (LDA) and the generalized-gradient approximation (GGA). For the bulk Be, the GGA results are slightly closer to the experimental values than are the LDA results. However, for the Be (0001) surface, neither the GGA nor LDA results explain the anomalously large expansion of the top layer observed in experimental low-energy electron-diffraction data.