A Molecular Dynamic Study of Complexing

In the molecular dynamics approaches to such systems, one of the principal novelties compared with the work on simpler systems such as KCI (Section 5.3.3) is the taking into account of anion polarizability, i.e., elimination of the hard-sphere approximation. Anions are by and large more polarizable than cations. The polarizability ce is proportional to rf. The radii of cations, formed by losing an outer electron, are smaller than those of the anions, which are formed by adding an electron. Hence, anions are mainly the ions affected when polarizability is to be accounted for. 

When an anion feels the field of a cation X, a dipole moment is induced into the anion. 

The calculations of Saboungi et al. on these NaCl-AlClj systems do not lack complexity. There are as many as ten terms of interaction. Thus, 

the first six terms refer to pair potential energies of interaction and the last four terms represent dipole-dipole or monopole-dipole interaction energies. 

Taking the anion as polarizable not only introduces these last terms but also reduces the repulsive energies used compared with those of earlier calculations that took a hard anion according to the mean spherical approximation. By hard anion is meant one that doesn t respond to being squeezed, i.e., is not polarizable. 

---