Figure shows the errors in E int using XSAPT + MBD in various Karlsruhe basis sets. Errors are unacceptably large in the absence of diffuse functions (even using def2-QZVP), which is not surprising, but minimal augmentation is sufficient to reduce the errors to ≤ 5 kcal/mol versus a CCSD(T)/CBS reference energy E int = -116.6 kcal/mol, 97 even at the double-ζ level. In quadruple-ζ basis sets with diffuse functions, the XSAPT + MBD errors are all ≤ 1.3 kcal/mol.