Basis set convergence of the coupled-cluster correction, δMP2CCSD(T): Best practices for benchmarking non-covalent interactions and the attendant revision of the S22, NBC10, HBC6, and HSG databases

In benchmark-quality studies of non-covalent interactions, it is common to estimate interaction energies at the complete basis set (CBS) coupled-cluster through perturbative triples [CCSD(T)] level of theory by adding to CBS second-order perturbation theory (MP2) a "coupled-cluster correction," delta(CCSD(T))(MP2), evaluated in a modest basis set. This work illustrates that commonly used basis sets such as 6-31G*(0.25) can yield large, even wrongly signed, errors for delta(CCSD(T))(MP2) that vary significantly by binding motif. Double-zeta basis sets show more reliable results when used with explicitly correlated methods to form a delta(CCSD(T*)(MP2-F12))-F12 correction, yielding a mean absolute deviation of 0.11 kcal mol-1 for the S22 test set. Examining the coupled-cluster correction for basis sets up to sextuple-zeta in quality reveals that delta(CCSD(T))(MP2) converges monotonically only beyond a turning point at triple-zeta or quadruple-zeta quality. In consequence, CBS extrapolation of delta(CCSD(T))(MP2) corrections before the turning point, generally CBS (aug-cc-pVDZ,aug-cc-pVTZ), are found to be unreliable and often inferior to aug-cc-pVTZ alone, especially for hydrogen-bonding systems. Using the findings of this paper, we revise some recent benchmarks for non-covalent interactions, namely the S22, NBC10, HBC6, and HSG test sets. The maximum differences in the revised benchmarks are 0.080, 0.060, 0.257, and 0.102 kcal mol(-1), respectively. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3659142]