AbstractComparison of the optimized geometries and SCF energies for the series XO 2+, XO2, XO 2−, XO 2−, with X = S,N shows thatd(S) functions cause larger bond shortening and energy drop thandfunctions centered on first‐row atoms. This is further emphasized on comparing the separate effects ofd(central atom) andd(O) functions for SO2and NO 2−, which are similar only for the first‐row molecule. Thed(S) functions are also essential for proper prediction of the OSO angles. The deformation densities calculated for each series and the corresponding X–O shared populations, change as expected on adding electrons first into σ* then into π* molecular orbitals. In the regions around nuclei the deformation densities express the behavior of the atomicsandpvalence orbitals or of their product inside their radial nodes. Introduction ofdfunctions causes substantial polarization effects. For X = N these are mostly local except in the bonding regions whered(N) andd(O) functions are somewhat interchangeable. However,d(S) functions induce also unique changes in the deformation density near O. They cause π and π′ charge migration from O to S and a σ flow in the opposite direction. These effects are largest for the hypervalent species. The unique populations of thed(S) functions are much larger than those ofd(N) andd(O) functions. The contribution ofd(S) functions to bonding is related to the larger amplitude at small radii of the atomic 3d(S) orbital as compared with that of 3d(N). The difference in amplitudes is related to penetration effects. Diffusepfunctions affect geometries and SCF energies of doubly, but not singly negative ions. However, they mostly describe the diffuse nonbonding clouds and do n