While there are certainly systematic errors in particular basis sets, we observe that the overall step-to-step fluctuations (both in E int and in its components) are very similar, across basis sets ranging in quality from 6-31+G(d) to def2-TZVPD. Systematic errors are different in various energy components and this leads to some error cancellation, such that E int computed at the XSAPT + ai D3/6-31+G(d) level differs from the corresponding def2-TZVPD value by only 1.9 ± 0.8 kcal/mol. This is just 2% of the ensemble-averaged interaction energy, E int = -87.0 kcal/mol. Error statistics for E int , with respect to XSAPT + ai D3/def2-TZVPD values that represent our best estimates for this system, are listed in Table along with timing data for various basis sets.