The small initial angle θ in PSI prevents us from further tilting towards ring D as strong steric repulsion effects or group overlaps might occur between the Chl and His molecules. Therefore, only about 3-4% of the spin-density is localized at His -in the electronic ground state (and 96-97% in the first excited state). These results are quite different from those obtained for tilted hinge models of PSII, where much larger delocalization (up to 40%/60% for θ ≈ 78 • ) was observed. This small delocalization is rather surprising considering the angles θ in both PSI molecular models. This small delocalization degree may probably be explained by the larger intermolecular distances R in the corresponding PSI complexes. Again, non-truncated models