Toward an ab initio Molecular Plasmonics

In the classification of models for metal-molecule electrodynamic coupling that we have done in Sec. 5.1, the model that we have described so far (a classical punctiform dipole close to a metal nanoparticle described as a continuous medium) is the simplest. While it has proven to be extremely useful, not only as a mean to grasp the basic physics of molecular plasmonics phenomena, but also to provide semi-quantitative and, sometimes, even quantitative results, it still remains a model empirical in nature. In this section we shall briefly describe models that goes beyond such an approach. 

1 Coupling the ab-initio Description of the Molecule with a Continuous Metal 

A model where an ab initio description of the molecule is coupled to a continuous description of the metal nanoparticle and the hosting matrix has been proposed in Ref. [51] and further developed in the course of the years [52, 54-58]. Here, we briefly present the basics of such model, referring to the original papers for details. 

The chromophore has been treated at the Hartree-Fock or density functional theory level (see Chapter 4), in the determination of both its ground state and its properties. This model for the molecule represents a remarkable progress in the accuracy of the description of the molecular chromophore compared to polarizable point dipole model. The solvation effects have been described with the PCM. More in details, the solvent is described as a continuum dielectric which occupies all the space free from the metal specimen and the molecule is hosted in a molecular shaped cavity inside such dielectric. 

From the point of view of the molecule, the electrostatic and/or electrod3mamics interactions with the metal nanoparticle enter directly an effective Hamiltonian. This is formally identical to the problem of a molecule in solution, when the solvent is described by a continuum dielectric [98]. Thus, the ground state wavefunction should minimize the molecular electronic density 

---

While the continuous body description of the metal is exploited, the molecule is treated atomistically by standard electronic structure techniques, such as time-dependent Hartree-Fock (TD-HF) or time-dependent density functional theory (TD-DFT) (see Sec. 4.4.2), and the electromagnetic interaction is included in the molecular Hamiltonian. This is a promising route not only to bypass inaccuracies related to the classical dipole model for the molecule, but also to go toward an ab initio molecular plasmonics. At present, this model has been explored mostly in the polarizable continuum model (PCM) group [51, 52, 54-58], but recently other implementations have been proposed [59]. 

---