Total energy calculations for prototype metals like Al and Ga on Si(001) 2×2, 2×1, and 1×2 surfaces predict important new structures and provide their bond strengths. It is estimated that at about 0.5-monolayer coverage the surface reconstruction is lifted and metallic dimers become stable. New low-energy Peierls-distorted structures provide a more natural explanation for the 5×2 and 3×2 LEED patterns. The reported trend in desorption energy as a function of coverage is reproduced, but more importantly specific conditions for observing this trend are obtained. © 1989 The American Physical Society.