New concepts of metallic bonding based on valence-bond ideas

From generalized-valence-bond calculations on numerous Li-atom clusters (Li_n and Li_n^+, n <- 13), we conclude that the optimum bonding involves singly occupied orbitals localized interstitially (in tetrahedra). Rules based on the calculations are used to predict low-energy isomers (leading for Li^+_(13) to low-symmetry structures that are significantly more stable than the icosahedron but retain local fivefold-symmetry axes) and are applied to infinite metallic systems.