Identification of Plasmons in Molecules with Scaled Ab Initio Approaches

Abstract

For the electronic excitations in metallic systems under periodic boundary conditions, momentum conservation and a uniform electron–electron interaction imply a clear distinction of plasmons and single-particle excitations. For finite molecular systems, this distinction is less clear, but excitations formed by a coherent superposition of elementary particle–hole transitions that show a collective oscillation of the transition electron density have nevertheless been identified as plasmons in molecules. To aid this distinction, a scaling approach [Bernadotte, S.; Evers, F.; Jacob, C. R. J. Phys. Chem. C 2013, 117, 1863] has recently been developed that is based on the observation that, in contrast to single-particle excitations, plasmonic excitation energies strongly depend on the electron–electron interaction. In this work, we adapt the proposed scaling scheme to ab initio models, specifically configuration interaction singles and the second-order algebraic diagrammatic construction scheme of the polarization propagator. The resulting approach is applied to a series of linear polyenes and the characterization based on the scaling method is confirmed by inspection of the eigenvector components, transition density patterns, and transition strengths.