Fig. 1.

a) Interaction between two Drude pairs iβ and jγ. The mobile Drude particles [carrying the charge q^δ] are represented by black filled circles whereas the Drude particle located at the corresponding atom are shown as open circles. The induced dipoles ${\mu }_{i\beta }^{ind}$ and ${\mu }_{j\gamma }^{ind}$ point from the central Drude particle to the mobile Drude particle. b) Interaction between a Drude pair iβ and a permanent charge of an atom jγ.