STRUCTURAL-PROPERTIES OF SODIUM MICROCLUSTERS (N=4-34) USING A MONTE-CARLO GROWTH METHOD

The structural and electronic properties of small sodium clusters are investigated using a distance-dependent extension of the tight-binding (Huckel) model and a Monte Carlo growth algorithm for the search of t he lowest energy isomers. The efficiency and advantages of the Monte C arlo growth algorithm are discussed and the building scheme of sodium microclusters around constituting seeds is explained in details. The p entagonal-based seeds (pentagonal bipyramids and icosahedral structure s) are shown to play an increasing role beyond n = 12. Optimized geome tries of Na(n) clusters are obtained in the range n = 4-21 and for n = 34. In particular, Na-20 is found to have C3 symmetry, hardly prolate with all axial ratios almost equivalent, whereas Na34 has D5h symmetr y and consists of a doubly icosahedral seed of 19 atoms surrounded by a ring of 15 atoms. Stabilities, fragmentation channels, and one-elect ron orbital levels are derived for the lowest isomers and shown to be characterized by a regular odd-even alternation. The present results a re in generally good correspondence with previous nuclei-based calcula tions when available. The global shapes of clusters, as well as the sh ape-induced fine structure splitting of the spherical electronic jelli um shell are found, with a few exceptions, to be also consistent with the ellipsoidal or spheroidal versions of the jellium model.