where we have introduced Ci = C i + ∇α i ∇ . Here, V 0 (r, r ) is the effective electrostatic interaction between two Coulomb charges, equal either to the bare Coulomb interaction for two proteins in a pure solvent, V 0 (r, r ) = 1/4πε w ε 0 |r -r |, or to the DH potential for two proteins in a monovalent electrolyte, V 0 (r, r ) = e -κ|r-r | /4πε w ε 0 |r -r |. For α i → 0, the result of Eq. ( ) coincides with the KS interaction as derived by Lund and Jönsson . It also resembles the recently calculated charge fluctuation Casimir interaction between dielectric spheres , but without the CR model.