Could you repeat that?

 

Ah, diffuse basis sets are important for the energies of anionic systems, but they may not even be that helpful for the geometries and cause a host of other problems. Frank Neese complains about this every now and then, and Tian Lu (aka Sobereva) also has a strong opinion. 

In short, don't put diffuse functions on hydrogen (ahem, 6-311++G**) and don't use diffuse functions for cations. Oh, and if you're going to benchmark a bunch of functionals, don't add diffuse functions instead of polarization; I will elaborate the roast later.