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This is used to specify a shielded Born-Meyer potential binary ionic clusters with anions and cations having the same magnitude charge. The atoms must be in the order cation, anion, cation, anion, etc. The energy is:

E = $\displaystyle \sum_{{i<j}}^{}$$\displaystyle \left[\vphantom{ \frac{q_iq_j}{r_{ij}} e^{-\gamma r_{ij}} + A_{ij}e^{-r_{ij}/\rho_{ij}} }\right.$$\displaystyle {\frac{{q_iq_j}}{{r_{ij}}}}$e-γrij + Aije-rij/ρij$\displaystyle \left.\vphantom{ \frac{q_iq_j}{r_{ij}} e^{-\gamma r_{ij}} + A_{ij}e^{-r_{ij}/\rho_{ij}} }\right]$.

The parameters that can be specified with PARAMS are γ, the magnitude of the ionic charges, a single value of ρ, assumed to be the same for all interactions, and A++, A and A+-, in order. Default values for KCl are set for all parameters except γ. Atomic units are assumed. Subroutines used: ions.