  We show that charge-sign-dependent asymmetric hydration can be modeled
accurately using linear Poisson theory but replacing the standard
electric-displacement boundary condition with a simple nonlinear boundary
condition. Using a single multiplicative scaling factor to determine atomic
radii from molecular dynamics Lennard-Jones parameters, the new model
accurately reproduces MD free-energy calculations of hydration asymmetries for
(i) monatomic ions, (ii) titratable amino acids in both their protonated and
unprotonated states, and (iii) the Mobley "bracelet" and "rod" test problems
[J. Phys. Chem. B, v. 112:2408, 2008]. Remarkably, the model also justifies the
use of linear response expressions for charging free energies. Our
boundary-element method implementation demonstrates the ease with which other
continuum-electrostatic solvers can be extended to include asymmetry.
