  We have developed and implemented an implicit electrolyte model in the Vienna
Ab initio Simulation Package (VASP) that includes nonlinear dielectric and
ionic responses as well as a nonlocal definition of the cavities defining the
spatial regions where these responses can occur. The implementation into the
existing VASPsol code is numerically efficient and exhibits robust convergence,
requiring computational effort only slightly higher than the original
self-consistent continuum solvation (SCCS) model. The nonlinear+nonlocal model
is able to reproduce the characteristic `double hump' shape observed
experimentally for the differential capacitance of an electrified metal
interface while preventing the `leakage' of the electrolyte into regions of
space too small to contain a single water molecule or solvation ion. The model
also gives a reasonable prediction of molecular solvation free energies as well
as the self-ionization free energy of water and the absolute electron chemical
potential of the standard hydrogen electrode. All of this, combined with the
additional ability to run constant potential density functional theory
calculations, should enable the routine computation of activation barriers for
electrocatalytic processes.
