Here, ρ i 1 is the electron density of the i 1 -th monomer, and ρ i 1 i 2 ...in is the electron density of the n-mer consisting of molecular fragments {i 1 , i 2 , . . . , i n }. In analogy to the manybody expansion of the total energy [see Eq. ( )], the second-order (two-body) correction to the electron density is given by ∆ρ