A ROBUST METHOD OF CALCULATING SURFACE ATOMIC GEOMETRY

We present a robust method for self-consistent solution of the Kohn-Sh am equations for both the electronic and ionic degrees of freedom. The solution is based on a supercell technique with local density functio nal pseudopotential theory. From a given initial set of atomic positio ns, the (self-consistent) charge density is calculated using Broyden's scheme. A conjugate gradient technique is then used to minimise the t otal energy by moving along the Born-Oppenheimer subspace and hence th e equilibrium geometry is achieved. The algorithm is applied to a numb er of III-V semiconductor substrate systems with and without various m etal overlayer coverages. Our results for clean GaAs(110) and InP(110) surfaces compare well with recent Car-Parrinello type calculations, a s well as those obtained via LEED analysis. In addition we present the results of our calculations for both GaAs(110) and InP(110) substrate s, with one and two ordered monolayer coverage of Bi and Sn. We find t hat in general the Bi overlayers practically remove the surface tilt a ngle, leaving a very small vertical shear on the adsorbate atoms. In c ontrast the Sn overlayer is characterised by a large vertical shear.