The solvation free energy is the free energy change of transferring a molecule from an ideal gas state to a solvent. If the solvent is water, we call the solvation free energy a hydration free energy (∆G hyd ). Solvation free energies tell us which phase a given molecule prefers in a multiphasic system and also provide information on how a solute behaves in different environments. In this work, our interest is in physical models which can be used to calculate activity coefficients and related properties from molecular simulations. Particularly, molecular simulations begin with a description of the energy and forces in a physical system as a function of the coordinates -what is known as a "force field" -and allow calculation of numerous physical properties from simulations of such systems . In addition to potentially providing predictions of various quantities like host-guest and protein-ligand binding affinities, distribution and partition coefficients, solvation free energies or other physical properties for design applications, comparison of such results to experiment provides a quantitative test of the underlying physical model or force field.