represents the interaction of the SCF density (ρ sol ) with the polarized continuum. For a PCM, this polarized electrostatic interaction is the entirety of the solvation energy. The permittivity function in eq. 1 is the one that defines the PCM but the equations outlined in this section can be solved for any smoothly varying function ε(r), subject only to numerical limitations. For example, it is not possible to use an absolutely sharp dielectric boundary (as in eq. 1), and for direct comparison to PCM results this boundary must be smoothed somewhat. In practice this must be done carefully. Too much smoothing causes the model to deviate from the sharp dielectric boundary that defines the PCM and other reaction-field solvation models that are predicated on the use of a sharp spherical boundary for the continuum. In the opposite limit, an absolutely sharp boundary causes wild oscillations in ρ iter (r), leading to convergence failure, unless the discretization grid is made extremely dense (at greatly increased cost). These features make the PEqS approach cumbersome in practice, even if its generality is appealing. HetPCM, on the other hand, is just as computationally simple and robust as any other PCM, requiring relatively inexpensive two-dimensional discretization of the cavity surface rather than three-dimensional discretization of all space. This makes it an attractive alternative to PEqS, provided that it realistically models the same phenomenology.