Figure 2 General performance of the B3LYP and ωB97M-V functionals with different basis sets for some of the reactions used in Experiments 1–3. All values are in kcal mol–1
compared to the energy of two or more constituting fragments at infinite separation, for example when a binding energy is calculated. The basis set of the united system and those of the separated fragments are intrinsically different, because the united system might benefit from an artificial increase of the basis set size due to the simultaneous presence of the basis functions of the monomers in the regions where they overlap. As a consequence of this artificial local increase in basis set size, the calculated binding energies can be fictitiously overestimated. BSSE is thoroughly discussed in experiment 5. Finally, we designed a review experiment on the choice of basis sets (experiment 6) using proton affinities, electron affinities, and ionization potentials. As part of this review experiment, we reiterate the general conclusion that basis sets that are specifically optimized for DFT calculations are superior to those optimized for wave function theory ones (not necessarily in terms of accuracy, but mainly in terms of an improved cost/accuracy ratio). These conclusions are in line with those found in most of the recent basis sets studies of DFT calculations.116–118 We need to point out, however, that there is no universal consensus on them, and while a recent study has shown that the Pople basis sets are not optimal for DFT,116 other authors119 have also pointed out that the Dunning’s family still performs satisfactorily.
Our final suggestion: Use particular care when choosing a basis set, so to minimize BSIE and BSSE. Possibly prefer basis sets that have been demonstrated to perform well for DFT, like the Jensen or Ahlrichs families. The Ahlrichs’ family is our own preferred choice,b since it has the additional advantage of being defined for most of the periodic table (all elements up to Rn), therefore avoiding issues that can arise when mixing basis sets belonging to different families, most importantly when dealing with organometallic complexes or heavy elements.120