The conventional choice for C-PCM is ζ = 0 in eq. 9, corresponding to the conductor limit of IEF-PCM. Other choices (chiefly ζ = 1/2) are sometimes encountered but will not be used here. For ε s 30, C-PCM is numerically indistinguishable from IEF-PCM. Even for smaller values of ε s the differences are modest, perhaps 1-2 kcal/mol in the electrostatic solvation energies of small molecules. 2.2. HetPCM. The IEF-PCM and C-PCM methods are defined by eq. 3 and the dependence on ε s is contained wholly within the factors f ε and fε (ζ) in eqs. 6 and 9, respectively. The HetPCM approach to be tested here, which was introduced in Ref. 41 for IEF-PCM and in Ref. 42 for C-PCM, consists in modifying this factor to use a different value of ε s for each atomic sphere. To fully specify the model, however, it is necessary to consider how we discretize the solute cavity surface.