The overlap integrals (2s mod 4f), (2p sigma mod 4f), and (2p pi mod 4f) have been calculated for the entire series of cubic lanthanide sesquioxides (C-Ln2O3) through analytical five-term (2s, 2p: O2-) and four-term (4f:Ln3+) ground-state wavefunctions. The set of distances used for C2 and C3i cation sites is reported for the first time. It has been found that only pi -type electron delocalisation is feasible within the fragment Ln3+-O2-; the respective overlap integrals vary from 0.12 to 0.03.