The X40×10 Halogen Bonding Benchmark Revisited: Surprising Importance of (<i>n</i>–1)d Subvalence Correlation

We have re-evaluated the X40×10 benchmark for halogen bonding using conventional and explicitly correlated coupled cluster methods. For the aromatic dimers at small separation, improved CCSD­(T)-MP2 “high-level corrections” (HLCs) cause substantial reductions in the dissociation energy. For the bromine and iodine species, (n–1)­d subvalence correlation increases dissociation energies and turns out to be more important for noncovalent interactions than is generally realized; (n–1)­sp subvalence correlation is much less important. The (n–1)­d subvalence term is dominated by core–valence correlation; with the smaller cc-pVDZ-F12-PP and cc-pVTZ-F12-PP basis sets, basis set convergence for the core–core contribution becomes sufficiently erratic that it may compromise results overall. The two factors conspire to generate discrepancies of up to 0.9 kcal/mol (0.16 kcal/mol RMS) between the original X40×10 data and the present revision.