Ionic charging free energies: Spherical versus periodic boundary conditions

Ionic charging free energies calculated by Ewald summation differ substanti ally from those calculated in spherical cluster calculations, with or witho ut the inclusion of a Born correction in the latter. Using Gauss' law, we d erive an electrostatic potential for ions in spherical clusters that involv es contributions only from the interior solvent. This "interior" potential agrees with the "P-summation" approach proposed by Hummer et al. [J. Phys. Chem. B 101, 3017 (1997)], and leads to charging free energies which agree, within simulation error, with those given by Ewald summation with finite-s ize corrections. The difference in charging free energies between this appr oach and the conventional cluster free energies including the Born correcti on is traced to the surface potential of water. (C) 1998 American Institute of Physics. [S0021-9606(98)50948-8].