Atomic point charge model

Fig. 6. Electrostatic multipole expanded interaction energy between two ethene molecules for 8 different orientations at R = Ua (ab initio results from refs. - ). Fits and are atom-atom (point charge) model fits to the ab initio data is a fit with the charges shifted from the nuclei, has the charges fixed on the nuclei. Also indicated is an empirical atom-atom potential for details see ref. ...

In contrast to the point charge model, which needs atom-centered charges from an external source (because of the geometry dependence of the charge distribution they cannot be parameterized and are often pre-calculated by quantum mechanics), the relatively few different bond dipoles are parameterized. An elegant way to calculate charges is by the use of so-called bond increments (Eq. (26)), which are defined as the charge contribution of each atom j bound to atom i. 

Williams, D. E., and J.-M. Yan. 1988. Point-Charge Models for Molecules Derived from Least-Squares Fitting of the Electric Potential. Adv. Atomic Mol. Phys. 23, 87. 

If separate D-RESP charge sets are fitted for every single one of the 36 frames, the standard deviation of the electrostatic field generated varies between 3.5 and 5% with respect to the full quantum reference. This accuracy is the best (in the least-squares sense) that can be obtained if the system is modeled with time-dependent atomic point-charges and represents the accuracy limit for a fluctuating point charge model of the dipeptide. 

To test our suspicion, we used a point charge model and ab initio calculated atomic charges in order to evaluate the electrostatic energy of 1,2-ds and 1,1-difluoro-ethylene. 

The approach is that adopted earlier for E b [Eq. (10.3)], using the point-charge model, which seems reasonable for describing interactions between distant atoms, at least in sigma systems. The charges and the geometries are the same as those used earlier for the total nonbonded interactions represented by Eq. (10.3). 

Note that in Eqs. (2.22) and (2.23) the dielectric constant e is subscripted. Although one might expect the best dielectric constant to be that for the permittivity of free space, such an assumption is not necessarily consistent with the approximations introduced by the use of atomic point charges. Instead, the dielectric constant must be viewed as a parameter of the model, and it is moreover a parameter that can take on multiple values. For use in Eq. (2.22),... 

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