New concepts of bonding in nonperiodic metallic systems

Using the generalized valence bond method, we have examined numerous Li atom clusters (up to 13 atoms). Our conclusion is that the optimum metallic bonding involves singly-occupied orbitals localized interstitially, e.g., in bond midpoints, triangular hollows, and tetrahedra. For Li₁₃⁺, the low energy isomers have local five-fold symmetry axes (as in an icosahedron) but lead to low overall symmetry. The guiding principle is that the optimum structures (denoted OPTET) optimize the number of tetrahedral hollows while keeping the sharing of vertices below a threshold. These OPTET structures are significantly more stable than the high-symmetry icosahedral, hcp-like and fcc-like clusters. Speculations are given on the relevance of these results for amorphous metallic systems.