Inverse power potential, bond distance

For the strong interactions dealt with in the last section, interatomic potentials are determined by overlap-dependent terms which have an inverse exponential variation with internuclear distance (R). At large separations the interactions are independent of overlap and may be represented by inverse powers ofR. There will be between these two an intermediate region, which encompasses the van der Waals minimum for non-bonded interactions, and we will take this to be contained in our definition of a weak-interaction. 

The contribution to the predicted electrostatic potential of the anisotropic atomic multipoles (Q , / > 0), which represent the lone pair and n-electron density, rapidly become less important as the distance between the molecules increases. This not only results from the inverse power of R increasing with I, but also from the cancellation between the contributions from different multipoles and different atoms. For example, there is generally an atomic dipole component along a bond that opposes the polarity implied by the atomic charges, as shown in the results of distributed multipole analyses (DMAs) of the azabenzenes. ° Thus, the accuracy gained by using a distributed multipole model is very dependent on the relative separation and orientation of the molecules, as well as the actual distribution of charge in the molecule. 

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