The recent experimental characterization of the Al-77 cluster by Ecker
et al. [Nature 387, 379 (1997)] detailed a fascinating onionlike stru
cture 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 e
xamined using density-functional methodology and basis sets with effec
tive core potentials. The stationary points located with this methodol
ogy were then further analyzed using full-electron basis sets of tripl
e-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,wit
h D-5h symmetry. The cationic, neutral, and anionic surfaces of Al@Al-
12 were explored. Structures similar to the Al,a stationary points wer
e found for the anionic Al@Al-12(-) cluster though in this case the st
aggered minimum had icosahedral symmetry and the eclipsed stationary p
oint was a true transition state. No atom-in-a-cage-type structure cou
ld be located for the cationic Al-13(+) cluster, but one minimum was f
ound for the neutral. That C-1 minimum is in astonishing agreement wit
h the core of Ecker's Al-77 cluster having center-to-surface bond leng
ths that range from 2.635 to 2.850 Angstrom and average 2.744 Angstrom
.