Trends in energies and geometric structures of neutral and charged aluminum clusters

The minimum energy geometric structures of Al-n, Al-n(-), and Al-n(+) ( 4 <= n <= 15) are predicted from the results of '' Tabu Search '' ( TS) global optimizations performed directly on the BPW91/LANL2DZ potential energy surface. In 24 of the 36 cases investigated, the TS delivered a lower energy structure than previously reported, in one case (Al-12(+) ) it failed to find the global minimum, and in the remaining 11 cases TS confirmed previous structures. All clusters (with 4 <= n <= 15) have the lowest spin state as their ground state except Al-4 (triplet), Al-4(+) (quartet), Al-7(-) (triplet), and maybe Al-5(+) (singlet and triplet are degenerate). The 20-electron Al-7(+) and 40electron Al-13(-) clusters are relatively stable compared to other clusters, on several criteria; to a lesser degree, Al-7, Al-12, and A(+)l(13) are also stable.