The equilibrium atomic geometry, energetic stability and chemical bonding for H and Cl passivation of the Si/Ge(001)-(2 x 1) surface have been studied by applying the ab initio pseudopotential method. In a recent theoretical work, a segregated structure in which Si occupies the second layer while Ge floats to the surface was found to be energetically favourable by 0.38 eV per dimer compared with the non-segregated Si-capped structure. Upon hydrogen passivation, the situation is reversed: the non-segregated structure becomes energetically favourable by 0.08 eV per dimer compared with the segregated structure. For chlorine passivation, this energy difference is 0.3 eV per dimer. For the non-segregated H(Cl):Ge(001)/Si system, the Si dimer becomes symmetric with the bond length elongated from 2.26 Angstrom to 2.39 Angstrom (2.42 Angstrom). For the segregated H(Cl):Ge(001)/Si system, the Ge dimer also becomes symmetric with the bond length elongated from 2.39 Angstrom to 2.45 Angstrom (2.46 Angstrom). (C) 2000 Elsevier Science B.V. All rights reserved.