Ab initio total energy calculations have been performed for Na, K and Rb adsorption on Ge(001)(2 x 1) surface. It was found that the adsorption site of AM is AM size dependent. Structural analysis showed that the Ge-Ge dimer bond becomes stronger with increasing AM size. As the coverage increases from 0.5 to 1 ML it turns out that no depolarization effect occurs upon Na adsorption, while this effect becomes more important with increasing AM size. We also found that for all adsorption systems investigated the germanium surface is metallic and semiconducting for the coverage of 0.5 and I ML, respectively. (c) 2005 Elsevier B.V. All rights reserved.