Electrostatic interactions short-range electron shell repulsion

To further illustrate the importance of coupling the electrostatic and short-ranged repulsion interactions, we consider the example of a dimer of polarizable rare gas atoms, as presented by Jordan et al. In the absence of an external electric field, a PPD model predicts that no induced dipoles exist (see Eq. [12]). But the shell model correctly predicts that the rare gas atoms polarize each other when displaced away from the minimum-energy (force-free) configuration. The dimer will have a positive quadrupole moment at large separations, due to the attraction of each electron cloud for the opposite nucleus, and a negative quadrupole at small separations, due to the exchange-correlation repulsion of the electron clouds. This result is in accord with ab initio quantum calculations on the system, and these calculations can even be used to help parameterize the model. ... 

The highly ionic nature of MgO means that quite accurate empirical potentials can be constmcted. The polarizable shell model potential is the most widely used for MgO and also for a wide range of other ionic materials. It is instmctive to discuss the main elements of this potential in order to understand the nature of interactions between the ions. The dominating contribution to the interaction is electrostatic and in the simplest approximation can be represented by associating a point charge (usually the formal charge) with each ion. In addition there is a short-range repulsive term due to the overlap of electron density between the ions (Born-Mayer) and a weakly attractive... 

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