The Hamiltonian matrix is represented in the n-particle basis. It can be diagonalized to give the coefficient vector C(κ) for the eigenvalue E κ (κ = 0, 1, 2, . . .). Note that Eq. ( ) resembles a truncated CI-singles and -doubles expansion with the neglect of charge transfer excitations between the monomers. This truncated CI-expansion refers to the intermolecular interaction only, i.e., the monomer states are fixed (zero-order states). Therefore, the FEH approach has frequently been termed perturbative . Two-particle excitations are implemented (see Section 2.3), but will not be considered in the applications below.