Long-range effects. The dispersion energy

With only two groups, A and B, the dispersion term (14.3.6) becomes 

Instead of using the transition densities explicitly, we revert to a matrix-element form of (14.4.2), noting that in second quantization (Problem 14.4) 

This operator, as usual, replaces orbital (pf by (f wherever it may appear in (destroying any terms in which is absent). Similar equations apply to system B and the orbital sets cpf used in 

The products in the numerator of (14.4.7) are reminiscent of those that occur in the polarization propagator used in (13.3.12). There is one product for molecule A, which would determine how a perturbation at point r, propagated to point r and a similar product for molecule B. But 

UJZ Pictorial interpretation of the dispersion energy. The energy formula (14.4.10) involves two coulomb interactions (solid lines), multiplied by two FDPs (wavy lines), integrated over all positions of the volume elements. Each FDP describes the propagation of a density fluctuation within the molecule. 

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