ISIM is package that simulates the thermodynamic ensemble of ions around a macromolecule using a grand canonical monte carlo scheme and simple hard sphere ion models. It is meant to provide an alternative mechanism to mean field approaches to allow the calculation of ion distributions around a highly charged molecule using a simple model that takes into account ion-ion correlations and steric interactions. The original source was created in the McCammon group at UCSD (http://mccammon.ucsd.edu/isim/). A Java interface is under development (see "Ion Simulator Interface" project on simtk.org)