Correlation functions coupled dipole moment systems

The dipole moment is the total dipole of the sample, p = Y.i Pi The correlation function describes the response of the system to the weakly coupled radiation field. The effects of the field are modeled by the response of the individual atoms or molecules unaffected by the weak coupling. The Hamiltonian describes the interaction of the field and matter (first-order perturbatiuon theory). The correlation function describes how the perturbed system approaches equilibrium. 

The fluctuation-dissipation theorem dictates that the response of a system to a weak perturbation is entirely determined by the equilibrium correlation of the fluctuating dynamic variable that couples with the field, i.e., ili p (f) = cmm (0> where cmm is the correlation function of the fluctuations of the total dipole moment in equilibrium ... 

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