First-principles calculations using density functional theory based on norm-conserving pseudopotentials have been performed to investigate the Mg adsorption on the Si(001) surface for 1/4, 1/2, and 1 monolayer (ML) coverages. For both 1/4 and 1/2 ML coverages it has been found that the most favorable site for the Mg adsorption is the cave site between two dimer rows consistent with the recent experiments. For the 1 ML coverage we have found that the most preferable configuration is when both Mg atoms on 2 × 1 reconstruction occupy the two shallow sites. We have found that the minimum energy configurations for 1/4 ML coverage is a 2 × 2 reconstruction while for the 1/2 and 1 ML coverages they are 2 × 1. © 2004 The American Physical Society.