We have performed two types of calculations to measure the covalent interactions in Aβ-Zn(II) coordination. NBO analysis, in which we have determined the delocalization energy, denoted as E(2), from each ligand to Zn(II). The most significant delocalizations are from the n O /n N lone pairs into the formally empty valence orbitals of Zn(II). In addition, we have calculated electron delocalization indexes between Zn(II) and O/N based on QTAIM , δ X-Zn with X=O, N, which is taken as a measure of the electron-pair sharing between two atoms. In Figure we show that there is a strong correlation between both quantities, suggesting a consistent trend in the degree of covalency along the different Zn(II)-ligand interactions. Imidazole ligands have the highest delocalization indexes and E(2) energies in tetrahedral and octahedral clusters. On the other hand, acetate ligands show lower delocalization indexes and E(2) energies, with water molecules in octahedral complexes having the lowest delocalization indexes and E(2) energies. In all cases, ligands in tetrahedral clusters show higher E(2) values and delocalization indexes (see Tables and).