To avoid confusion with mechanistic core-shell models that are used in force fields to mimic polarizability note that we refer here to a volumetric shell model. Our shell is the outer region of a sphere that has a (much) higher weight for computing volume averages than the sphere's core region, contrasting force field-related core-shell models in which a massless shell particle is tethered to a heavy core particle via a harmonic spring. Our electrostatics-based approach, specifically with the volumetric shell model(s), has however similarities with spherical cavity Onsager models used in apparent surface chargedielectric continuum solvation models such as the conductor-like screening model (COSMO). These models are, for example, used to accurately predict partition coefficients of solutes in solvents on the basis of density functional theory calculations. The electrostatic potential within a host material can be readily determined via the Hartree potential, V H , from DFT calculations with the Vienna Ab initio Simulation Package 26