similar models (see, e. g., Refs. 22, 36–38, 48), where ξ is chosen as the unit of energy, we shall use throughout β = 1. The reason is that we aim to compare the cases of organic and metallic electrodes among themselves, and their bandwidths (4 ξ within our model) significantly differ. On the other side, we do not expect substantial differences between the resonance integrals β corresponding to a molecule attached to organic and metallic electrodes, which should be of the order of a few electron volts. Basically, the same can also be expected for the Coulomb contact interaction J . Concerning the size N of the extended molecule we note the following. The main interest of the present paper is not the IRLM itself. Rather, we aim to unravel what one can learn from the results for this model for theoretical approaches to electron transport in real systems based on feasible ab initio quantum-chemical calculations able to accurately treat strong electron correlations. With the presently available computational resources it is unlikely that accurate calculations can include more than a few electrode layers. Within the present model, this would correspond to at most N ≈ 12. Therefore, it would make little sense to examine the results for the IRLM up to the largest size ( N ≈ 26), which can be tackled within full CI calculations by calculating the Green’s function of the isolated molecule G r i from Eq. (18) based on the Lanczos algorithm [24, 48–53], nor to carry out a finite scaling analysis of the numerical results to extract the limit N → ∞ . For the sizes of interest, the straightforward exact numerical diagonalization can be and has been employed to compute the exact Green’s function G r $_{i}$, Eq. (18), within full CI calculations. To model a (chemisorbed) molecule linked to organic electrodes, one should consider that the resonance integrals ξ and β are of comparable magnitudes, and this results in large values of the hybridization parameter Γ = 2 β $^{2}$/ξ . For illustration, in this subsection we shall examine the case where they are equal, ξ = β (= 1). First, we want to benchmark the approximation of the sufficiently large extended molecule presented in Sect. II against a well established exact result, namely the case where the molecular level is at resonance with the electrode Fermi energy ( α = ε$_{F}$ ). There, similar to the cases of the noninteracting resonant level ( J ≡ 0) and of the single-electron transistor [20, 21, 47], the conductance of the IRLM reaches the unitary limit G = G 0 . Physically, this 12 A. Case of organic electrodes