Harmonic field representation of dielectric response

In Sections (16.3) and (16.4) we have seen that the description of motion along the reaction coordinate in the two electronic states involved contains elements of displaced harmonic potential surfaces. Here we develop this picture in greater 

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Our starting point is the expression for the reorganization energy E, Eq. (16.71). We use it in a more general form that expresses the energy that will be released when we start from a fluctuation in the nuclear polarization about a given charge distribution and let the system relax to equilibrium  

Here the index I is used to denote a particular charge distribution (i.e. a particular electronic state of the system). The displacement field P/(r) represents a charge distribution p/(r) according to the Poisson equation V T i = npi. In (16.79) D(r ) and the associated p(r) represent a fluctuation in the nuclear polarization, defined by the equilibrium relationship between the nuclear polarization and the displacement vector (cf. Eqs (16.14) and (16.15)) 

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