Theoretical cluster calculations for the Si(100) surface show that the use of doubly occupied orbital wavefunctions, such as the closed-shell Hartree–Fock (HF), lead to an asymmetric dimer description of the surface. The inclusion of electron correlation produces a symmetric dimer description with a ground state ∠1.0 eV below the minimum of the HF buckled dimer. There are two low-lying states of the symmetric dimer (a singlet and a triplet) with very different geometries. Energy minimization calculations indicate that a (2×1) structure is favored over a c(2×2) structure. We also report ionization potentials for surface and Si(2p) core electrons that are consistent with current experimental data.