Nevertheless, the significance of nonadditive dispersion for large vdW complexes prompted a search for a less ad hoc way to describe this phenomenon. To that end, we have explored the use of XSAPT in conjunction with the MBD model developed for DFT by Tkatchenko and co-workers. It builds upon an earlier dispersion model developed by Tkatchenko and Scheffler, 135 which is sometimes called "vdW-TS" in the condensed-matter DFT literature. The theory behind both MBD and its predecessor has been reviewed recently. Briefly, the vdW-TS method determines an in situ (or "effective") value for C 6,a by means of a volume-scaling relationship,