  Theories of solvation free energies often involve electrostatic potentials at
the position of a solute charge. Simulation calculations that apply cutoffs and
periodic boundary conditions based on molecular centers result in
center-dependent contributions to electrostatic energies due to a systematic
sorting of charges in radial shells. This sorting of charges induces a
surface-charge density at the cutoff sphere or simulation-box boundary that
depends on the choice of molecular centers. We identify a simple solution that
gives correct, center-independent results, namely the radial integration of
charge densities. Our conclusions are illustrated for a Lennard-Jones solute in
water. The present results can affect the parameterization of force fields.
