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 C-2 ((1)Sigma(+)(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.