Soft-Sphere Continuum Solvation in Electronic-Structure Calculations.

We present an implicit solvation approach where the interface between the quantum-mechanical solute and the surrounding environment is described by a fully continuous permittivity built up with atomic-centered "soft" spheres. This approach combines many of the advantages of the self-consistent continuum solvation model in handling solutes and surfaces in contact with complex dielectric environments or electrolytes in electronic-structure calculations. In addition it is able to describe accurately both neutral and charged systems. The continuous function, describing the variation of the permittivity, allows to compute analytically the nonelectrostatic contributions to the solvation free energy that are described in terms of the quantum surface. The whole methodology is computationally stable, provides consistent energies and forces, and keeps the computational efforts and runtimes comparable to those of standard vacuum calculations. The capabilitiy to treat arbitrary molecular or slab-like geometries as well as charged molecules is key to tackle electrolytes within mixed explicit/implicit frameworks. We show that, with given, fixed atomic radii, two parameters are sufficient to give a mean absolute error of only 1.12 kcal/mol with respect to the experimental aqueous solvation energies for a set of 274 neutral solutes. For charged systems, the same set of parameters provides solvation energies for a set of 60 anions and 52 cations with an error of 2.96 and 2.13 kcal/mol, respectively, improving upon previous literature values. To tackle elements not present in most solvation databases, a new benchmark scheme on wettability and contact angles is proposed for solid-liquid interfaces and applied to the investigation of the stable terminations of a CdS (112̅0) surface in an electrochemical medium.