The interaction of atomic F, Cl and group-III metal atoms with cluster models simulating the Si(111) and Ge(111) surfaces, and atomic H and O with cluster models simulating the Cu(100) and Ag(100) surfaces have been studied at the SCF and CI levels by using ab initio pseudopotentials. Several methods and strategies used to include electronic correlation effects are discussed. In particular, the analysis of correlation effects in terms of non-dynamical, dynamical and core-valence contributions provides a useful instrument for the qualitative understanding of the chemisorption bond.