Nonlocal polarizability densities and van der Waals interactions

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Abstract
The van der Waals interaction energy of a molecular pair is derived within a simple physical model that represents the distribution of polarizable matter in each molecule by the use of nonlocal polarizability densities. In the model the field due to the instantaneous polarization of one molecule polarizes the second molecule nonlocally, thereby producing a reaction field that acts on the first molecule. The resulting energy change depends upon the averaged product of the fluctuating polarization at different points in the first molecule; by the fluctuation–dissipation theorem, this product is related to the imaginary part of the nonlocal polarizability density. The model gives the van der Waals energy as an integral of a contracted tensor product of two dipole propagators and the imaginary-frequency nonlocal polarizabilities of the interacting molecules. The validity of the model is established by comparison with results from second-order perturbation theory.