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