Bond dipole model

Williams, D. E. 1988. Representation of the Molecular Electrostatic Potential by Atomic Multipole and Bond Dipole Models. J. Comp. Chem. 9, 745. 

Surprisingly, the atomic dipole model (D) is nearly as good as the atomic charge (M) model. If only dipoles are used, it may be better to locate them in the bonds. The table shows that the bond dipole model (BD) gives a better representation of the electric potential for most of the molecules considered. Of course, at least one monopole is needed if the molecule is an ion and therefore has a net charge. 

Table 12 Goodness-of-Fit of PD Atomic Multipole Expansions and Bond Dipole Models ...

Williams showed that an unrestricted bond dipole model gave erratic results for the bond dipole directions. To pursue the goals of chemical reasonableness and transferability, it seems appropriate to restrict the direction of the bond dipoles. The most natural direction is along the bond. Table 12 shows that restricted bond dipole models represent the electric potential about as well as monopole models. The two models also have nearly the same number of parameters. So the choice between these two models is a matter of convenience. If long distance interaction is considered, as in crystals, use may be made of the fact that dipole-dipole energy converges much faster than monopole-monopole energy. However, as mentioned above, if ions are considered, monopole interactions are still needed. Table 13 summarizes values of restricted bond dipole moments. 

D. E. Williams, /. Comput. Chem., 9, 745 (1988). Representation of the Molecular Electrostatic Potential by Atomic Multipole and Bond Dipole Models. See also, D. E. Williams, in Reviews in Computational Chemistry, Vol. 2, K. B. Lipkowitz and D. B. Boyd, Eds., VCH Publishers, New York, 1991, pp. 219-271. Net Atomic Charge and Multipole Models for the Ab Initio Molecular Electric Potential. 

L. D. Barron, in Molecular Light Scattering and Optical Activity, Cambridge University Press, Cambridge, U.K., 1982, pp. 317-321. P. L. Polavarapu, Mol. Phys., 49, 645 (1983). A Comparison of Bond Moment and Charge Flow Models for Vibrational Circular Dichroism Intensities. J. R. Escribano, T. B. Freedman, and 1- A. Nafie, /. Phys. Chem., 91, 46 (1987). A Bond-Origin-Independent Formulation of the Bond Dipole Model of Vibrational Circular Dichroism. 

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