Chemisorption properties of atomic Cs on the Ga-rich GaAs (100) (2 x 1), (2 x 2), and beta(4 x 2) surfaces are investigated using ab initio self-consistent restricted open shell Hartree-Fock total energy calculations with Hay-Wadt effective core potentials. The effects of electron correlation have been included using many-body perturbation theory through second order, with the exception of beta(4 x 2) symmetry, due to computational limitations. The semiconductor surface is modeled by finite sized hydrogen saturated clusters. The effects of surface relaxation and reconstruction have been investigated in detail. We report on the energetics of chemisorption, charge population analysis, highest occupied molecular orbital-lowest unoccupied molecular orbital gaps, and consequent possibilities of metallization. The most stable chemisorbed site is found to be the trough site for the Ga9As9H25 cluster, with the adatom at a distance of 3.796 Angstrom from the surface at the MP2 level. The highest occupied molecular orbital-lowest unoccupied molecular orbital gap, in this case is reduced by 0.724 eV due to Cs adsorption. At the Hartree-Fock level, the most stable chemisorbed site is also the trough site for the Ga19As15H39 cluster, with a reduction in the gap of 1.403 eV.