Density functional theory is applied within a supramolecular approach to the study of the guest−host interactions in [Fe(bpy)3]2+@Y and their influence on the structural, energetic, and 57Fe Mssbauer spectroscopy properties of the encapsulated [Fe(bpy)3]2+ complex in the low- and high-spin states. The structures of the isolated complex and the supramolecular model used for [Fe(bpy)3]2+@Y were optimized in both spin-states using different generalized gradient approximation (PBE, HCTH, OLYP) and hybrid (B3LYP*, O3LYP) functionals. The results obtained are consistent with one another and show that, in either spin-state, the structure of [Fe(bpy)3]2+ shrinks and distorts upon encapsulation. Still, the structural changes experienced by the complex in a given spin-state remain limited, especially in that they do not lead to a substantial variation of the 57Fe quadrupole splitting, whose calculated values are in very good agreement with avalaible experimental data. The decomposition of the guest−host interaction energy into its electrostatic, Pauli and orbital contributions shows that the bonding between the complex and the supercage is more electrostatic than covalent. The ability of modern functionals to accurately describe the interactions explains the remarkable consistency of the results obtained with the various functionals. In particular, although the functionals perform very differently for the determination of the high-spin/low-spin energy difference ΔEHLel in [Fe(bpy)3]2+ and [Fe(bpy)3]2+@Y, they consistently predict that the encapsulation entails a destabilization of the high-spin state with regard to the low-spin state of Δ(ΔEHLel) = 2500 cm−1. Using for [Fe(bpy)3]2+ the CASPT2 value of ΔEHLel = 3700 cm−1 [ Pierloot, K.; Vancoillie, S. J. Chem. Phys. 2006, 125, 124303; Pierloot, K.; Vancoillie, S. J. Chem. Phys. 2008, 128, 034104], we obtain for the high-spin/low-spin energy difference in [Fe(bpy)3]2+@Y, a best ab initio estimate of ΔEHLel = 6200 cm−1.