Interaction between polar molecules small particles

2 Interaction between polar molecules (smal-1 particles) 

In most cases, the dispersion forces are dominant except for small and highly polar molecules like water. These expressions are in good agreement with experimental data and when the studied interaction arises between dissimilar molecules 1 and 2, C i2 is close to the geometric mean of C and Cw. 

The important case of water does not obey this empiric law because it is an highly polarised molecule. 

The London theory assumes that molecules have only one single ionization potential and therefore one absorption frequency and it does not handle the effect of the solvent to the particle interaction potential. 

In 1963, McLachlan [7, 8] gave a new expression for the Van der Waals free energy of two small particles (1 and 2) in a medium (3)  

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INTERACTION BETWEEN POLAR MOLECULES (SMALL PARTICLES) 51... 

Finally, the whole system (molecule + metal nanoparticle) can be treated atomistically via TD-DFT or other quantum chemical methods. The interaction between the metal nanoparticle and the molecule are treated on the same foot as the intra-molecule and intra-nanoparticle ones. This method is therefore able to include much more than just the electrodynamics coupling, as it can include mutual polarization, chemical bonding, charge transfers (also in excited states). On the down side, at present this approach is limited to very small metal particles (a few tens of atoms, a few nm in size). Moreover, electrodynamics coupling is limited to the quasi-static limit, as standard molecular Hamiltonian includes only non-retarded Coulombic potential. Nevertheless, this method represents a fully ab initio approach to molecular plasmonics. 

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