IDACs (γ ∞ ), as defined in the Introduction, tell us how far from ideality a mixture is when the concentration of the solute is infinitely small. They are widely used as input for engineering models, such as for prediction of liquid-vapor equilibria, and they can be calculated from solvation free energies (Eq. 2). The natural logarithm of γ ∞ is proportional to the difference between the free energy of solvation of a solute i in a given solvent (∆G solv i ) and the free energy of solvation of the solute molecule in its pure bulk phase (∆G self solv i , free energy of "self-solvation") plus the ratio between the densities of the solvent and the pure solute: