Collectivity in the optical response of small metal clusters

The question of whether the linear absorption Spectra of metal clusters can be interpreted as density oscillations (collective "plasmons") or can only be understood as transitions between distinct molecular states is still a matter of debate for clusters with only a few electrons. We calculate the p hotoabsorption spectra of Na, and Na-5(+) comparing two different methods: quantum fluid dynamics and time-dependent density functional theory. The ch anges in the electronic structure associated with particular excitations ar e visualized in "snapshots" via transition densities. Our analysis shows th at even for the smallest clusters, the observed excitations can be interpre ted as intuitively understandable density oscillations. For Na-5(+), the im portance of self-interaction corrections to the adiabatic local density app roximation is demonstrated.