Probing the basis set limit for thermochemical contributions of inner-shell correlation: balance of core-core and core-valence contributions^*

The inner-shell correlation contributions to the total atomisation energies of the W4-17 computational thermochemistry benchmark have been determined at the CCSD(<i>T</i>) level near the basis set limit using several families of core correlation basis sets, such as aug-cc-pCVnZ (<i>n</i> = 3–6), aug-cc-pwCVnZ (<i>n </i>= 3–5) and nZaPa-CV (<i>n </i>= 3–5). The three families of basis sets agree very well with each other (0.01 kcal/mol RMS) when extrapolating from the two largest available <i>n</i>: however, there are considerable differences in convergence behaviour for the smaller basis sets. nZaPa-CV is superior for the core-core term and awCVnZ for the core-valence term. While the aug-cc-pwCV(<i>T</i>+d)Z basis set of Yockel and Wilson is superior to aug-cc-pwCVTZ, further extension of this family proved unproductive. The best compromise between accuracy and computational cost, in the context of high-accuracy computational thermochemistry methods, is CCSD(<i>T</i>)/awCV{<i>T</i>,<i>Q</i>}Z, where the {<i>T</i>,<i>Q</i>} notation stands for extrapolation from the awCVTZ and awCVQZ basis set pair. For lower-cost calculations, we recommend a previously proposed combination of CCSD-F12b/cc-pCVTZ-F12 and CCSD(<i>T</i>)/pwCVTZ(no f). While in first-row molecules core-valence correlation on average accounts for over 90% of the inner-shell contribution, in second-row molecules core-core contributions may become important, particularly in systems like P₄ and S₄ with multiple adjacent second-row atoms.