Atomistic dipole moments and polarizabilities of Na_N clusters, N=2-20

The atomic-level response of Na_N clusters, N=2-20, to a small static external electric field is studied using a method that decomposes the total cluster dipole moment and polarizability into contributions from nonoverlapping atomic volumes. The atomic dipole moments and polarizabilities are, in turn, partitioned into the so-called dipole and charge-transfer components. The former characterizes a dielectric type of a response, whereas the latter represents a metallic type of a response. Analysis of the atomic polarizabilities points to their strong dependence on the site, or location, of the atoms within the structure of the clusters. Surface atoms have larger polarizabilities than the interior ones. Overall, the fraction of the charge-transfer component of the averaged atomic polarizabilities is an increasing function of the cluster size. The charge-transfer component is also responsible for the structure/shape driven variations in the atomic polarizabilities. The anisotropy of the total polarizabilities correlates with the shape anisotropy of the clusters.