CCSD(T)/CBS atomic and molecular benchmarks for H through Ar.

We extrapolate to the coupled cluster single and double excitation and the perturbative triples (CCSD(T))/complete basis set (CBS) limit with a sequence of optimized n-tuple-ζ augmented polarization augmented (nZaPa) basis sets (n = 4, 5, 6, and 7) for 115 species representing the first two rows of the Periodic Table. The species include the entire set of atoms, positive and negative atomic ions, homonuclear diatomic molecules, and hydrides. The benchmark set also includes the rare gas dimers, polar molecules such as oxides and fluorides, and a few transition states for chemical reactions. The CCSD correlation energies agree with available CCSD-F12b/3C(FIX) values to within ±0.18 mEh root-mean-square (rms) deviation. The (T) components agree to within ±0.10 mEh and the total CCSD(T) correlation energies to within ±0.26 mEh or 0.1% rms deviation, which is probably the better measure, since the largest deviation is 0.43 mEh or 0.13%. These CBS limits can now be used as benchmarks to calibrate more approximate calculations using smaller basis sets. The sequence of basis sets provides data on convergence patterns for each component of the correlation energy.