3.1.2. Convergence Errors for Double-Hybrid Functionals. Convergence error statistics with respect to the CBS limit are summarized for double-hybrid functionals in Table . Inclusion of MP2 correlation means that we expect the convergence behavior of these functionals to resemble that of correlated wave function methods, which is generally much slower than DFT, and the convergence behavior of MP2 itself is also examined in Table . We observe that MP2 convergence errors in Pople basis sets and in def2-SVP are large with CP correction than without, which in general can simply be ascribed to the fact that these basis sets are not appropriate for a correlated wave function method. A more incisive explanation is gleaned by examining the raw data for S66 interaction energies computed at the MP2 level (Table ), which reveals that MP2 severely underestimates ∆E int for the dispersion-bound subset of S66 when these basis sets are employed. Since the absence of CP correction leads to a larger interaction energy, there is a partial error cancellation that is upset by CP correction. This behavior contrasts with the systematically smaller convergence errors obtained when CP correction is applied to GGA and singly-hybrid functionals, and it sets up a competition between that behavior and the behavior of MP2 correlation in the case of double-hybrid functionals, when very small basis sets are employed. This leads to slightly less systematic convergence behavior than what was observed for GGA and singly-hybrid functionals; CP correction generally reduces the convergence errors, but by amounts that are somewhat functional-dependent.