Plasmons in Molecules

The classification of electronic excitations in molecules and molecular nanostructures plays an important role when tailoring materials with desired properties. One example of such a class of excitations are plasmons. Plasmons appear in solid-state physics, where they are characterized as density oscillations in an electron gas that are driven by the restoring forces associated with the electromagnetic field induced by the density oscillations themselves. Here, we investigate how this concept can be transferred to molecular systems by performing a step-by-step analysis, starting from three-dimensional bulk systems and ending with molecules. On the basis of this analysis, we propose to scale the electron-electron interaction in quantum-chemical response calculations in order to identify plasmons in molecules. This approach is illustrated for molecular chains and clusters. Our results show that the concept of plasmons is still applicable for extended molecular systems and demonstrate that the proposed scaling approach provides an easy way of characterizing electronic excitations.