Basis set convergence of high-order coupled cluster methods up to CCSDTQ567 for a highly multireference molecule

We examine the basis-set convergence of post-CCSD(T) contributions to the dissociation energy of the highly multireference C₂(¹Σ_g ⁺) molecule. At the infinite basis set limit we obtain: CCSDT–CCSD(T)/cc-pV{6,7}Z = −2.268, CCSDT(Q)–CCSDT/cc-pV{6,7}Z = 3.420, CCSDTQ–CCSDT(Q)/cc-pV{5,6}Z = −1.151, CCSDTQ(5)–CCSDTQ/cc-pV{Q,5}Z = 0.412, CCSDTQ5–CCSDTQ(5)/cc-pV{T,Q}Z = −0.053, CCSDTQ5(6)–CCSDTQ5/cc-pV{D,T}Z = 0.060, CCSDTQ56–CCSDTQ5(6)/cc-pVTZ(1d) = 0.003, CCSDTQ56(7)–CCSDTQ56/cc-pVTZ(1d) = 0.002, and CCSDTQ567–CCSDTQ56(7)/cc-pVTZ(1d) = 0.001 kcal/mol. These post-CCSD(T) contributions add to 0.427 kcal/mol. Including the CCSD(T)/CBS energy, inner-shell, scalar relativistic, spin-orbit, DBOC, and ZPVE corrections from W4.3 theory results in a CCSDTQ567/CBS D₀ value of 144.08 kcal/mol, which agrees to within overlapping uncertainties with the experimental ATcT value of 144.006 ± 0.06 kcal/mol.