I want to adapt my existing MD simulation so that it can handle these three species.

For the interactions of the ions I thought that I would use the coulomb potential, but I don't know what sort of potential I should use between water and the ions.

It doesn't have to be super accurate, I'm not doing ab initio stuff. Also, am I correct in choosing coulomb between the force field between the ions?

I am representing all 3 species at a molecular level of granularity.

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    $\begingroup$ You could try using a Debye–Hückel type potential ... $\endgroup$ – John Rennie Sep 20 '13 at 9:43

There is a wide range of molecular-level water models to choose from. The simplest is the SPC ("single point charge") model which is a rigid three-site model with a Coulomb and a short-range Lennard-Jones interaction term. This model reproduces structural properties such as the radial distribution function reasonably well. More sophisticated models give more realistic descriptions of e.g. the dielectric properties of liquid water.

Yes, the ion-ion (and ion-water) interactions are Coulombic, but you also need a short range repulsion (for example a Lennard-Jones potential) to exclude unphysical configurations where point charges of opposite sign pile up on each other. A recent example for constructing ionic force fields in water is the paper Ionic Force Field Optimization.

A classic reference is the book Computer Simulation of Liquids by Allen and Tildesley.

  • $\begingroup$ I second that excellent text. $\endgroup$ – Declan Aug 25 '15 at 9:41

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