We have calculated the atomic structure of alkali metals overlayers on Si (001) at two different coverages. It is concluded that at 1/2 monolayer coverage, the quasihexagonal site proposed by Levine is favorable if the lattice is allowed to relax. The long bridge site along the dimerization direction is somewhat less stable. Although the top site above one of the dimer forming Si atoms is found to be energetically unfavorable at the coverages investigated, an asymmetric buckling of the surface does lead to a substantial lowering of the total energy. These findings are discussed in the context of recent scanning tunneling microscopy observations. At the monolayer coverage, our calculations predict a puckered structure. It is also shown that the 2 × 1 reconstruction is stable at least up to 1 monolayer coverage of alkali metals. The implications of the 2X3 low-energy electron diffraction structure for adsorption sites are briefly discussed. © 1990, American Vacuum Society. All rights reserved.