Model potential (MP) parameters and valence basis sets were systematically determined for the main group elements Li through Rn, For alkali and alkaline-earth metal atoms, the outermost core (n - 1)p electrons were treated explicitly together with the ns valence electrons. For the remaining atoms, only the valence ns and np electrons were treated explicitly. The major relativistic effects at the level of Cowan and Griffin’s quasi-relativistic Hartree-Fock method (QRHF) were incorporated in the MPs for all atoms heavier than Kr. The valence orbitals thus obtained have inner nodal structure. The reliability of the MP method was tested in calculations for X-, X, and X+ (X = Br, I, and At) at the SCF level and the results were compared with the corresponding values given by the numerical HF (or QRHF) calculations. Calculations that include electron correlation were done for X-, X, and X+ (X = Cl and Br) at the SCDI level for AS(2) at the CASSCF and MRSDCI levels. These results were compared with those of all-electron (AE) calculations using the well-tempered basis sets. Close agreement between the MP and AE results was obtained at all levels of the treatment.