Ab initio calculations for structure and temperature effects on the polarizabilities of Na-n (n <= 20) clusters

We present a rigorous, nb initio theoretical calculation of the dependence of Na cluster polarizability on cluster size, up to 20 atoms, obtained by c ombining ab initio pseudopotentials with a gradient-corrected density funct ional. Using molecular dynamics, we find that for clusters as small as nine atoms, a multitude of degenerate isomers exists even at T=0. By calculatin g the polarizability of these isomers, we reproduce the generally decreasin g nature of the measured polarizability curve, as well as its dips at "magi c" numbers corresponding to closed electronic shells. Moreover, we find tha t the effect of a finite temperature on the cluster structure suffices to a ccount for most of the quantitative discrepancy between theory and experime nt.