Ab initio calculations on the ground electronic states of Cr2 and Mo2 show that f-type polarization functions in the orbital bases contribute 0.93 and 1.4 eV to the SCF energies of these molecules at their respective Re. These contributions vary rapidly with the internuclear separation and have a strong effect on the calculated Re, 0.06 A in Cr2 and 0.08 A in MO2. Previous studies have indicated that this effect can be expected to carry over to and may even be magnified considerably in correlated wavefunctions. The effects of f functions must be present, to a significant degree, in all systems involving d bonding. Results of previous theoretical calculations on Cr2 and Mo2 are analyzed in light of the present finding and the results of recent spectroscopic experiments. © 1983.