The adsorption of In on the Si(0 0 1)-(2 x 2) surface is studied, based upon ab initio pseudo-potential calculations. Of the two possible orientations for the In ad-dimers adsorption sites on this Surface, viz. In ad-dimers parallel to the underlying Si dimer (model 1) and In ad-dimers orthogonal to the underlying Si dimer (model 11), we find model I to be energetically more favourable by I eV/ad-dimer. For model 1, the Si-Si dimer becomes symmetric with an elongated bond length of 2.41 Angstrom, and the In-In dimer is also essentially symmetric with a bond length of 2.74 Angstrom. The Si-Si dimer bond length is somewhat bigger than the covalent diameter for Si while the In-In bond length is somewhat shorter than the atomic diameter for In. The calculated surface electronic structure and orbital bonding results support the available angle-resolved photoemission and scanning tunnelling spectroscopy measurements. (C) 2002 Elsevier Science B.V. All rights reserved.