We report results of ab initio pseudopotential density functional calculations for the geometry and energetics of X:Ge/Si(0 0 1)-(2 x 1) and X:Ge/Si(0 0 1)-(3 x 1) for a monolayer deposition of Ge in the presence of passivating species X (H and Cl). For non-passivated cases, the non-segregated Ge-capped structure was found to be energetically favourable by 0.49 eV per dimer for (2 x 1) and 0.90 eV for (3 x 1) compared to the segregated structure in which Si atoms float to the surface while Ge atoms occupy the second layer. For the hydrogen passivation, we found no considerable energy difference between the non-segregated and segregated structures for the (2 x 1) and also for the (3 x 1) surface reconstruction. However, for the chlorine passivation, We found the ion-segregated (2 x 1) surface structure to be 0.23 eV energetically favourable compared with its segregated structure, with this energy difference increasing to 0.48 eV for the (3 x 1) reconstruction. We also compare our calculated structural parameters with some of the available results. (C) 2002 Elsevier Science B.V. All rights reserved.