Al12 and the Al@Al12 clusters

The recent experimental characterization of the Al-77 cluster by Ecker et al. [Nature 387, 379 (1997)] detailed a fascinating onionlike structure whose inner core we report on here. The clusters Al-12 and Al@Al-12 have been studied in detail. Potential-energy surfaces have been examined using density-functional methodology and basis sets with effective core potentials. The stationary points located with this methodology were then further analyzed using full-electron basis sets of triple-zeta plus double polarization size. Two stationary points have been located on the Al-12 surface, a staggered minimum of D-5d symmetry and a saddle point of Hessian order three in an eclipsed conformation,with D-5h symmetry. The cationic, neutral, and anionic surfaces of Al@Al-12 were explored. Structures similar to the Al,a stationary points were found for the anionic Al@Al-12(-) cluster though in this case the staggered minimum had icosahedral symmetry and the eclipsed stationary point was a true transition state. No atom-in-a-cage-type structure could be located for the cationic Al-13(+) cluster, but one minimum was found for the neutral. That C-1 minimum is in astonishing agreement with the core of Ecker's Al-77 cluster having center-to-surface bond lengths that range from 2.635 to 2.850 Angstrom and average 2.744 Angstrom.