We have investigated Rb adsorption on the Si(100) surface for 0.5 and 1 monolayer coverages using the total energy method with norm-conserving pseudopotentials. For 2 x 1 reconstruction at 1 ML coverage symmetrized dimers are found to be energetically more favorable. On the other hand, half a ML coverage is found to have symmetrical dimers only for the most stable adsorption model. All possible surface-adatom configurations have been considered in the calculations to find which adsorption sites are energetically favored. In addition to the structural properties, the interface is investigated electronically for the work function and surface states. The results are discussed and compared with the existing experimental findings. (c) 2005 Elsevier B.V. All rights reserved.