Appearance of the bulk motif in Al clusters

We have performed an unbiased search for the lowest-energy structures of medium-sized aluminum clusters Al(n) (n=19-26) using a genetic algorithm (GA) coupled with a tight-binding interatomic potential. Structural candidates obtained from our GA search were further optimized using density functional theory. It is found that the double icosahedron is not the most stable structure for Al(19) but serves as the core for Al(20) and Al(21). The lowest-energy structures of Al(n) are found to undergo a transition to an aluminum bulk motif above Al(23). In particular, the lowest-energy structure of Al(26) is almost a fragment of the bulk face-centered-cubic crystal except for the stacking fault at the bottom layer. Anion clusters were also studied.