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 i n t e r s t i t i a l l y , e.g., in bond midpoints, triangular hollows, and tetrahedra. For LiTR, the low energy isomers have local f ive-fold symmetry axes (as in an icosa~edron) 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 signif icantly more stable than the high-symmetry icosahedral, hcp-like and fcc-l ike clusters. Speculations are given on the relevance of these results for amorphous metallic systems.