Giant plexcitonic effects in small plasmonic nanoparticles from an ab initio GW-BSE approach*

Plasmonic nanocatalysts can harvest photon energy to populate excited electronic state manifolds and drive nonequilibrium chemistry through highly tunable surface plasmon resonances. However, typical theoretical treatments neglect many-electron interactions, which are crucial for the accurate description of the rich excited-state dynamics in other low-dimensional systems. Here, we study realistically-sized AgPd nanoparticles with an ab initio GW plus Bethe-Salpeter equation (BSE) approach, allowing us to naturally capture both plasmons and bound excitons on the same footing. For this we have developed a toolkit of techniques, with broad applicability to GW-BSE calculations on large isolated systems. Notably, we use an extrapolation procedure which exploits the separability of the GW self-energy into exchange and correlation contributions that converge at different rates, and a method to accelerate the inversion step in obtaining the electron-hole correlation function, based on the low-rank form of the exchange interaction in a plane-wave basis. We find that, despite the metallic nature of bulk AgPd alloys, small AgPd nanoparticles host a continuum of strongly bound excitons, and show strong modulation of the highly-absorbing plasmon states due to excitonic effects. We also find a non-trivial spatial distribution of the excited-state electron and hole charge density in the presence of dopants, which are expected to impact chemical dynamics.