Lithium clusters Lin (n=5 to 20) were studied by Kohn–Sham theory with local spin density and gradient-corrected energy functionals. We used a Tabu Search algorithm for structure optimization. The lowest energy Lin isomers that we found fall in two categories: (i) the pentagonal bipyramid, icosahedron, and related structures which are typical of most pair potentials, and (ii) structures containing centered square antiprisms which are reminiscent of the bulk bcc structure and have two characteristic peaks in the pair distribution function, one near 2.60 Å and the other near 3.05 Å. Calculated isomer energies and vibrational frequencies suggest that, at room temperature, many cluster sizes should show liquidlike behavior or coexistence of multiple isomers. The number of unpaired electrons “M” as a function of cluster size “n” generally alternates between 0 (singlet) and 1 (doublet), but some cluster sizes display anomalous spin magnetic moments M(n); they are M(13)=5, M(16)=2, M(17)=3, and M(18)=2. The Li7, Li8, Li19, and Li20 clusters are particularly stable: they each have a very compact structure and a shape consistent with the ellipsoidal jellium model.