The effect of F cav (r) in eq. 28 is to make sure that ε ≈ 1 for points near the vdW cavity surface, which is constructed using the atomic radii defined in eq. 23. We take L = 0.5 Å and α = 4/L, as for the single-sphere cavity, so that the switching function is centered L/2 = 0.25 Å outside of the vdW surface as shown in Fig. .