Plasmon Character Index: An Accurate and Efficient Metric for Identifying and Quantifying Plasmons in Molecules

Plasmons, which are collective and
coherent oscillations of charge
carriers driven by an external field, play an important role in applications
such as solar energy harvesting, sensing, and catalysis. Conventionally,
plasmons are found in bulk and nanomaterials and can be described
with classical electrodynamics. In recent years, plasmons have also
been identified in molecules, and these molecules have been utilized
to build plasmonic devices. As molecular plasmons can no longer be
described by classical electrodynamics, a description using quantum
mechanics is necessary. In this Letter, we develop a quantum metric
to accurately and efficiently identify and quantify plasmons in molecules.
A number, which we call the plasmon character index (PCI), can be
calculated for each electronic excited state and describes the plasmonicity
of the excitation. PCI is developed from the collective and coherent
excitation picture in orbitals and shows excellent agreement with
the predictions from scaled time-dependent density functional theory
but is vastly more computationally efficient. Therefore, PCI can be
a useful tool in identifying and quantifying plasmons and will inform
the rational design of plasmonic molecules and nanoclusters.