Another case where an analytic solution to Poisson's equation is available is Kirkwood's solution for an arbitrary point multipole in a spherical cavity, embedded in an isotropic medium. To use this model, ρ sol (r) is represented by a multipole expansion (up to = 20 for these examples), then the analytic result is applied for each spherical harmonic function. In Table , we present results for two polar diatomic molecules and one polyatomic ion, using spherical cavities with R = 5.5-6.5 Å so that there is no escaped charge. Because the solvation energies for the neutral species are quite small, the PEqS grid spacing is set to ∆x = ∆y = ∆z = 0.05 Å in order to provide well-converged results.