Potential function for ions

One of the simplest and therefore computationally less expensive potential functions for ion-water consists of the sum of long-range Coulorabic electrostatic interactions plus short-range dispersion interactions usually represented by the Lennard-Jones potential. This last term is a combination of 6 and 12 powers of the inverse separation between a pair of sites. Two parameters characterize the interaction an energetic parameter e, given by the minimum of the potential energy well, and a size parameter a, that corresponds to the value of the pair separation where the potential energy vanishes. The 6-th power provides the contribution of the attractive forces, while repulsive forces decay with the 12-th power of the inverse separation between atoms or sites. 

Potential functions for ion-water pair interactions composed by Coulombic and short-range interactions are investigated. Short-range interaction parameters are determined from fitting to the DFT curve for the dissociation of monohydrates in the C2v symmetry. We found that other exponents such as 10-3, 9-2, 6-4, and 4-1, added to Coulombic terms, give better fits to the ab initio results than those obtained when 12-6 functions are used. The new functions yield better approximations to the DFT repulsive forces and to the minimum of the potential well. 

Polarizable model potential function for ion-methanol systems ... 

---