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Real-Time Description of the Electronic Dynamics for a Molecule close to a Plasmonic Nanoparticle

The optical properties of molecules close to plasmonic nanostructures greatly differ from their isolated molecule counterparts. To theoretically investigate such systems in a Quantum Chemistry perspective, one has to take into account that the plasmonic nanostructure (e.g., a metal nanoparticle - NP) is often too large to be treated atomistically. Therefore, a multiscale description, where the molecule is treated by an ab initio approach and the metal NP by a lower level description, is needed. Here we present an extension of one such multiscale model [Corni, S.; Tomasi, J. {\it J. Chem. Phys.} {\bf 2001}, {\it 114}, 3739] originally inspired by the Polarizable Continuum Model, to a real-time description of the electronic dynamics of the molecule and of the NP. In particular, we adopt a Time-Dependent Configuration Interaction (TD CI) approach for the molecule, the metal NP is described as a continuous dielectric of complex shape characterized by a Drude-Lorentz dielectric function and the molecule- NP electromagnetic coupling is treated by an equation-of-motion (EOM) extension of the quasi-static Boundary Element Method (BEM). The model includes the effects of both the mutual molecule- NP time-dependent polarization and the modification of the probing electromagnetic field due to the plasmonic resonances of the NP. Finally, such an approach is applied to the investigation of the light absorption of a model chromophore, LiCN, in the presence of a metal NP of complex shape.