B3LYP/cc-pVTZ+1 geometries are close enough to their CCSD(T)/VQZ+1 counterparts that their use does not cause a major effect on the final computed result. There is one notable exception to this rule for the molecules considered here: Cl2, for which the B3LYP/VTZ+1 bond distance of 2.0130 Å is quite different from its CCSD(T)/VQZ+1 counterpart, 1.9972 Å. (The experimental value[71] is 1.9879 Å.) While B3LYP and B3PW91 on the whole tend to produce essentially identical geometries and harmonic frequencies[68], it has been argued previously[72] that the B3PW91 functional may be somewhat more reliable for systems with high electron density; and in fact, the B3PW91/VTZ+1 bond distance of 1.9912 Å is much closer to the CCSD(T)/VQZ+1 and experimental value.
Even so, the use of a B3LYP/VTZ+1 reference geometry still does not affect the computed De by more than 0.14 kcal/mol. We conclude that CCSD(T) geometry optimizations, which will become fairly costly for larger molecules, can safely be replaced by B3LYP calculations, which can also serve for obtaining zero-point energies.