It is useful to rearrange Eq. ( ) into the form of an EDA. This will contain some pairwise-additive parts but also a many-body contribution. To identify and isolate the latter, we need to define pairwise SAPT energy components. Unlike XSAPT components, these are based on isolated-monomer wavefunctions with no embedding charges, so the total SAPT energy components are strictly pairwise additive. Define these to be