Exchange energies (Fig. ) span a similarly wide range and here the diffuse functions are important, to prevent severe underestimation of E exch caused by the tails of the anion's wave function. That said, the def2-SVPD value (E exch = 144.6 kcal/mol) is only about 1 kcal/mol different from the def2-QZVPD value (E exch = 143.8 kcal/mol). Often, the sum E elst + E exch is easier to interpret than either of these energy components alone, . Using heavy-augmented basis sets, this sum appears to converge at the triple-ζ level, where the difference with respect to def2-QZVPD is only 0.8 kcal/mol, which once again suggests that diffuse functions on hydrogen are not essential at the triple-ζ level. With a full complement of diffuse functions, the def2-SVPD basis set can be substituted for the triple-ζ one with 1 kcal/mol loss in accuracy.