Removal of the diffuse functions also has a smaller effect at the triple-and quadruple-ζ levels. Whereas in the double-ζ case the use of diffuse functions is are absolutely essential in order to obtain even semi-quantitative results, at the triple-or quadruple-ζ level it appears that minimal augmentation is sufficient. We conclude that diffuse functions added to double-ζ basis sets are partially compensating for the overall incompleteness of the monomer basis set, which becomes much less of an issue at the triple-ζ level. We will carefully document the importance of diffuse functions throughout this work, because in our experience many quantum chemistry users are extremely reluctant to include these functions, presumably for reasons of cost.