Restoring charge asymmetry in continuum electrostatics calculations of hydration free energies

Implicit solvation models are symmetric with respect to charge inversion. On the other hand, explicit solvent simulations show significant asymmetry in hydration free energies with respect to charge reversal. It has not been obvious how implicit solvation models can be modified in a general way to address this serious shortcoming. We solve this problem by using the induced surface charge density (ISCD) obtained from a boundary element solution of the Poisson equation to derive a simple customized correction for the Born radius of each atom. This simple correction restores the charge asymmetry and reproduces the hydration free energies from free energy perturbation calculations with a high degree of fidelity. The induced surface charge density profile of an atom essentially provides a measure of the average preferred orientation of a water molecule in the first solvation shell, thereby allowing a proper assignment of the dielectric boundary. We refer to these modified radii as ISCD-corrected Born radii.