Structural calculations for bulk as.

The valence-electron contribution to the total energy of simple- cubic and rhombohedral arsenic has been calculated using local- density- functional theory and a scalar-relativistic version of the linear augmented-plane-wave method. The results at normal pressure show that the observed rhombohedral (A7) structure is favored energetically over the cubic phase by about 0.07 eV/atom. The calculated values for the lattice parameter (a = 4.084angstroms), rhombohedral angle (a = 55.9D), and internal displacement parameter (u = 0.2294) are in excellent agreement (~0.3%, 2%, and 0.8%, respectively) with experiment. The cubic-rhombohedral energy difference, which is due to a Peierls-type mechanism at normal volume, is found to decrease at reduced volumes, leading to a stable high-pressure (~190 kbar) cubic phase for V/V(o)