Modeling the near-field effect on molecular excited states using the discrete interaction model/quantum mechanical method.

This article focuses on extending the discrete interaction model/quantum mechanical (DIM/QM) method to explicitly include local field effects in the sum-over-state formalism of time-dependent density functional theory (TDDFT) for studying strong coupling between molecular excited states and plasmonic metal nanoparticles. The authors demonstrate that commonly used two-state models are insufficient when the plasmon resonance is detuned from molecular excitations, requiring many electronic states for accurate description. They compare DIM/QM results with the simpler coupled-dipole model (CDM), finding CDM adequate for single molecule–single nanoparticle systems but inadequate for multiple molecules or nanoparticle junctions due to complex local field distributions. The study highlights that coupling strength depends strongly on the molecule's position relative to plasmonic "hot spots," that vibronic effects slightly reduce coupling by redistributing oscillator strength, and that an atomistic description of the nanoparticle cavity is essential to capture the local electromagnetic environment governing molecule–plasmon interactions.