Another choice of alchemical states involves partitioning the total solute-solvent interaction into terms for disjoint subsets of atoms. Here, we partition the full DS - into a head group, h, and a tail group, t, with h corresponding to the charged atomic sites, SO 4 CH - 2 , and t corresponding to the remaining 11-carbon chain having no partial charges. We describe SO 4 CH - 2 as the hydrophilic limit of DS -since it is a small ion and lacks any nonpolar tail group. Note that when simulating SO 4 CH - 2 , we set all LJ parameters on t to 0, but h and t still interact through the intramolecular force field (including stretching, bending, and torsional forces, but not including intramolecular LJ forces).