Instantaneous dipolar fluctuations

The leading R dispersion term is often described as the correlation in the instantaneous dipolar fluctuations in the charge density of the two molecules. 

A tacit assumption in the derivation of Eq. 6.18 is that the response of one molecule to the distorted charge distribution of the other is effectively instantaneous. If the intermolecular separation is small enough, this approximation does not affect the calculation of V(z), but at some distance of separation its effect is significant. In broad terms, this distance is reached when the time required for the fluctuating dipolar field of one molecule to... 

Molecules that do not possess permanent dipole posses a non-zero instantaneous dipole moment because of fluctuations caused for instance by electromagnetic radiation fields. Then instantaneous values of dipolar moment must be taken into... 

In general, jump models have been developed that permit instantaneous jumps between two or three nonequivalent molecular configurations. A more general formulation permitting jumps between several configurations is discussed in terms of lattice models. Jump models are particularly useful, in comparison to difiusion models, in that fluctuations in the intemuclear distance that may occur with the molecular motions can be accomodated easily. With the difiusion models, an avers e intemuclear distance r is used to calculate the dipolar contribution to relaxation parameters via Eq. (8). With jump models, the intemuclear distance can be explicitly specified for each jump state in this case the is removed from Eq. (8), and the distance... 

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