Two reconstruction structures are proposed for Bi adsorption on Si(100
) based on a first-principles local-density-functional molecular-clust
er study employing total energies and atomic forces. Bi dimers, rather
than monomers, are predicted to form the basic structure on the Si(10
0) surface. At high coverage, Bi adsorption leads to the breaking of S
i dimers, which in turn removes the reconstruction of Si(100)2 X 1. Ou
r results appear to explain recent results of scanning tunneling micro
scopy experiments.