Notably, "solvation energy" is the free energy needed to transfer a solute molecule from a solvent to "vacuum" (gas phase). This energy can supplement the intramolecular energy in vacuum calculated in molecular mechanics. Thus, the needed atomic solvation parameters were initially derived from water-gas partition data. However, the dielectric properties of proteins and lipid bilayers are much more similar to those of nonpolar solvents than to vacuum. Newer parameters have thus been derived from octanol-water partition coefficients or other similar data. Such parameters actually describe "transfer" energy between two condensed media or the "difference" of two solvation energies.