Figure 2.

Cartoons illustrating how a buffer layer surrounding a spherical cavity can be introduced. In (a), the reaction field computed at the boundary between explicit and implicit solvents r=R_c diverges. To avoid this singularity, it is common to compute reaction field for particles within R_c using a slightly larger radius R_c+τ. The result is the creation of a vacuum layer of thickness τ between explicit and implicit solvents. For nonzero τ, this mathematical model differs strongly from the underlying physical system. In (b), the buffer layer is filled with solvent; water molecules within radius R_c experience reaction field generated by the molecules within R_c+τ. Even for large τ, the model correctly represents homogeneous solvent.